py: Add stack check to mp_iternext, since it can be called recursively.
py: Add stack check to mp_iternext, since it can be called recursively.

Eg, builtin map can map over a map, etc, and call iternext deeply.

Addresses issue #1294.

vitiral/micropython,supergis/micropython,suda/micropython,deshipu/micropython,bvernoux/micropython,PappaPeppar/micropython,chrisdearman/micropython,oopy/micropython,orionrobots/micropython,jmarcelino/pycom-micropython,deshipu/micropython,ganshun666/micropython,stonegithubs/micropython,hiway/micropython,TDAbboud/micropython,ernesto-g/micropython,ceramos/micropython,alex-march/micropython,MrSurly/micropython-esp32,vitiral/micropython,xhat/micropython,turbinenreiter/micropython,lowRISC/micropython,puuu/micropython,slzatz/micropython,cloudformdesign/micropython,xuxiaoxin/micropython,jlillest/micropython,martinribelotta/micropython,micropython/micropython-esp32,adamkh/micropython,torwag/micropython,slzatz/micropython,trezor/micropython,rubencabrera/micropython,rubencabrera/micropython,martinribelotta/micropython,ryannathans/micropython,turbinenreiter/micropython,jlillest/micropython,utopiaprince/micropython,adamkh/micropython,kostyll/micropython,HenrikSolver/micropython,jmarcelino/pycom-micropython,pramasoul/micropython,tobbad/micropython,skybird6672/micropython,henriknelson/micropython,tralamazza/micropython,torwag/micropython,xuxiaoxin/micropython,EcmaXp/micropython,hosaka/micropython,cnoviello/micropython,tdautc19841202/micropython,danicampora/micropython,dinau/micropython,vriera/micropython,tobbad/micropython,suda/micropython,cloudformdesign/micropython,cnoviello/micropython,ganshun666/micropython,cnoviello/micropython,oopy/micropython,praemdonck/micropython,turbinenreiter/micropython,cwyark/micropython,pozetroninc/micropython,ahotam/micropython,EcmaXp/micropython,vitiral/micropython,stonegithubs/micropython,galenhz/micropython,firstval/micropython,ceramos/micropython,blazewicz/micropython,supergis/micropython,suda/micropython,MrSurly/micropython,alex-march/micropython,ahotam/micropython,redbear/micropython,noahwilliamsson/micropython,xuxiaoxin/micropython,vriera/micropython,jmarcelino/pycom-micropython,drrk/micropython,kostyll/micropython,dhylands/micropython,blazewicz/micropython,ganshun666/micropython,redbear/micropython,ryannathans/micropython,ryannathans/micropython,xhat/micropython,skybird6672/micropython,MrSurly/micropython-esp32,dxxb/micropython,hosaka/micropython,mgyenik/micropython,trezor/micropython,firstval/micropython,cwyark/micropython,lowRISC/micropython,trezor/micropython,pfalcon/micropython,Peetz0r/micropython-esp32,blazewicz/micropython,selste/micropython,infinnovation/micropython,puuu/micropython,pramasoul/micropython,SHA2017-badge/micropython-esp32,micropython/micropython-esp32,dinau/micropython,heisewangluo/micropython,suda/micropython,adafruit/circuitpython,cloudformdesign/micropython,kerneltask/micropython,SHA2017-badge/micropython-esp32,cwyark/micropython,dxxb/micropython,vitiral/micropython,ericsnowcurrently/micropython,toolmacher/micropython,stonegithubs/micropython,chrisdearman/micropython,lowRISC/micropython,noahchense/micropython,tuc-osg/micropython,trezor/micropython,Timmenem/micropython,tdautc19841202/micropython,suda/micropython,drrk/micropython,xyb/micropython,ericsnowcurrently/micropython,matthewelse/micropython,oopy/micropython,jmarcelino/pycom-micropython,tralamazza/micropython,jlillest/micropython,alex-robbins/micropython,tralamazza/micropython,deshipu/micropython,emfcamp/micropython,skybird6672/micropython,cwyark/micropython,adafruit/circuitpython,heisewangluo/micropython,swegener/micropython,noahchense/micropython,feilongfl/micropython,alex-march/micropython,orionrobots/micropython,ernesto-g/micropython,chrisdearman/micropython,misterdanb/micropython,ruffy91/micropython,omtinez/micropython,adafruit/circuitpython,tobbad/micropython,vriera/micropython,TDAbboud/micropython,turbinenreiter/micropython,tobbad/micropython,adafruit/circuitpython,MrSurly/micropython,mgyenik/micropython,misterdanb/micropython,jlillest/micropython,HenrikSolver/micropython,blmorris/micropython,toolmacher/micropython,supergis/micropython,MrSurly/micropython,adamkh/micropython,skybird6672/micropython,martinribelotta/micropython,MrSurly/micropython-esp32,supergis/micropython,martinribelotta/micropython,heisewangluo/micropython,PappaPeppar/micropython,feilongfl/micropython,galenhz/micropython,kostyll/micropython,dinau/micropython,xuxiaoxin/micropython,misterdanb/micropython,dmazzella/micropython,ruffy91/micropython,blmorris/micropython,misterdanb/micropython,mgyenik/micropython,feilongfl/micropython,MrSurly/micropython-esp32,mhoffma/micropython,jmarcelino/pycom-micropython,mgyenik/micropython,blmorris/micropython,xhat/micropython,henriknelson/micropython,vriera/micropython,stonegithubs/micropython,orionrobots/micropython,toolmacher/micropython,redbear/micropython,dhylands/micropython,mpalomer/micropython,ericsnowcurrently/micropython,feilongfl/micropython,orionrobots/micropython,rubencabrera/micropython,mhoffma/micropython,ahotam/micropython,infinnovation/micropython,tobbad/micropython,tuc-osg/micropython,Peetz0r/micropython-esp32,xhat/micropython,noahchense/micropython,emfcamp/micropython,hosaka/micropython,matthewelse/micropython,kerneltask/micropython,lbattraw/micropython,ceramos/micropython,skybird6672/micropython,pozetroninc/micropython,ahotam/micropython,Peetz0r/micropython-esp32,mianos/micropython,kerneltask/micropython,selste/micropython,supergis/micropython,danicampora/micropython,toolmacher/micropython,ChuckM/micropython,mpalomer/micropython,TDAbboud/micropython,lbattraw/micropython,danicampora/micropython,micropython/micropython-esp32,turbinenreiter/micropython,firstval/micropython,ChuckM/micropython,alex-robbins/micropython,noahchense/micropython,ganshun666/micropython,henriknelson/micropython,AriZuu/micropython,neilh10/micropython,tdautc19841202/micropython,oopy/micropython,alex-march/micropython,tdautc19841202/micropython,ernesto-g/micropython,blmorris/micropython,puuu/micropython,pozetroninc/micropython,selste/micropython,AriZuu/micropython,emfcamp/micropython,slzatz/micropython,blazewicz/micropython,pfalcon/micropython,AriZuu/micropython,danicampora/micropython,AriZuu/micropython,puuu/micropython,tdautc19841202/micropython,ceramos/micropython,noahwilliamsson/micropython,PappaPeppar/micropython,cnoviello/micropython,EcmaXp/micropython,dxxb/micropython,matthewelse/micropython,lbattraw/micropython,emfcamp/micropython,galenhz/micropython,TDAbboud/micropython,mhoffma/micropython,tuc-osg/micropython,rubencabrera/micropython,blazewicz/micropython,emfcamp/micropython,hiway/micropython,lbattraw/micropython,toolmacher/micropython,HenrikSolver/micropython,neilh10/micropython,dmazzella/micropython,AriZuu/micropython,neilh10/micropython,adafruit/micropython,torwag/micropython,dinau/micropython,redbear/micropython,blmorris/micropython,matthewelse/micropython,dhylands/micropython,cloudformdesign/micropython,ernesto-g/micropython,PappaPeppar/micropython,TDAbboud/micropython,ernesto-g/micropython,hosaka/micropython,HenrikSolver/micropython,ruffy91/micropython,mianos/micropython,heisewangluo/micropython,hiway/micropython,EcmaXp/micropython,xyb/micropython,utopiaprince/micropython,kerneltask/micropython,hosaka/micropython,mpalomer/micropython,misterdanb/micropython,mhoffma/micropython,noahchense/micropython,SHA2017-badge/micropython-esp32,slzatz/micropython,omtinez/micropython,dxxb/micropython,Timmenem/micropython,selste/micropython,bvernoux/micropython,dinau/micropython,mpalomer/micropython,adafruit/circuitpython,ceramos/micropython,omtinez/micropython,bvernoux/micropython,adafruit/micropython,micropython/micropython-esp32,noahwilliamsson/micropython,bvernoux/micropython,tuc-osg/micropython,pozetroninc/micropython,adafruit/micropython,drrk/micropython,matthewelse/micropython,cloudformdesign/micropython,pramasoul/micropython,adafruit/micropython,praemdonck/micropython,firstval/micropython,lowRISC/micropython,swegener/micropython,adamkh/micropython,adafruit/circuitpython,neilh10/micropython,mianos/micropython,utopiaprince/micropython,ganshun666/micropython,adamkh/micropython,hiway/micropython,torwag/micropython,redbear/micropython,dmazzella/micropython,neilh10/micropython,MrSurly/micropython,puuu/micropython,torwag/micropython,matthewelse/micropython,SHA2017-badge/micropython-esp32,feilongfl/micropython,xuxiaoxin/micropython,dhylands/micropython,selste/micropython,rubencabrera/micropython,Peetz0r/micropython-esp32,infinnovation/micropython,orionrobots/micropython,swegener/micropython,mpalomer/micropython,mianos/micropython,omtinez/micropython,alex-march/micropython,drrk/micropython,alex-robbins/micropython,lowRISC/micropython,tuc-osg/micropython,alex-robbins/micropython,micropython/micropython-esp32,galenhz/micropython,xyb/micropython,adafruit/micropython,drrk/micropython,HenrikSolver/micropython,kerneltask/micropython,omtinez/micropython,galenhz/micropython,vitiral/micropython,ericsnowcurrently/micropython,praemdonck/micropython,henriknelson/micropython,pozetroninc/micropython,infinnovation/micropython,swegener/micropython,utopiaprince/micropython,pramasoul/micropython,ericsnowcurrently/micropython,deshipu/micropython,chrisdearman/micropython,utopiaprince/micropython,pfalcon/micropython,ryannathans/micropython,slzatz/micropython,bvernoux/micropython,pfalcon/micropython,firstval/micropython,swegener/micropython,infinnovation/micropython,cnoviello/micropython,ruffy91/micropython,xyb/micropython,xyb/micropython,praemdonck/micropython,kostyll/micropython,trezor/micropython,pfalcon/micropython,martinribelotta/micropython,ChuckM/micropython,MrSurly/micropython,lbattraw/micropython,Timmenem/micropython,Peetz0r/micropython-esp32,ChuckM/micropython,ryannathans/micropython,oopy/micropython,hiway/micropython,stonegithubs/micropython,mgyenik/micropython,mianos/micropython,ahotam/micropython,danicampora/micropython,vriera/micropython,Timmenem/micropython,mhoffma/micropython,dxxb/micropython,ChuckM/micropython,SHA2017-badge/micropython-esp32,praemdonck/micropython,deshipu/micropython,heisewangluo/micropython,dhylands/micropython,cwyark/micropython,alex-robbins/micropython,MrSurly/micropython-esp32,dmazzella/micropython,jlillest/micropython,noahwilliamsson/micropython,noahwilliamsson/micropython,Timmenem/micropython,tralamazza/micropython,chrisdearman/micropython,PappaPeppar/micropython,henriknelson/micropython,kostyll/micropython,pramasoul/micropython,xhat/micropython,EcmaXp/micropython,ruffy91/micropython