extmod/modbtree: Cleverly implement "for key in btree:" syntax.
extmod/modbtree: Cleverly implement "for key in btree:" syntax.

I.e. make it work like btree.keys(), while still not using a separate
iterator type.

HenrikSolver/micropython,deshipu/micropython,adafruit/circuitpython,Timmenem/micropython,henriknelson/micropython,blazewicz/micropython,pramasoul/micropython,trezor/micropython,torwag/micropython,ryannathans/micropython,chrisdearman/micropython,pramasoul/micropython,SHA2017-badge/micropython-esp32,tobbad/micropython,mhoffma/micropython,dxxb/micropython,adafruit/micropython,adafruit/micropython,turbinenreiter/micropython,pfalcon/micropython,jmarcelino/pycom-micropython,alex-march/micropython,alex-march/micropython,selste/micropython,micropython/micropython-esp32,pozetroninc/micropython,PappaPeppar/micropython,pozetroninc/micropython,hiway/micropython,kerneltask/micropython,tuc-osg/micropython,tralamazza/micropython,henriknelson/micropython,SHA2017-badge/micropython-esp32,ganshun666/micropython,mhoffma/micropython,matthewelse/micropython,blazewicz/micropython,toolmacher/micropython,dmazzella/micropython,jmarcelino/pycom-micropython,bvernoux/micropython,Timmenem/micropython,lowRISC/micropython,adafruit/circuitpython,deshipu/micropython,hiway/micropython,bvernoux/micropython,henriknelson/micropython,chrisdearman/micropython,alex-robbins/micropython,emfcamp/micropython,jmarcelino/pycom-micropython,adafruit/circuitpython,puuu/micropython,HenrikSolver/micropython,adafruit/circuitpython,ganshun666/micropython,PappaPeppar/micropython,mhoffma/micropython,hosaka/micropython,tralamazza/micropython,blazewicz/micropython,Timmenem/micropython,alex-march/micropython,Timmenem/micropython,torwag/micropython,cwyark/micropython,ryannathans/micropython,oopy/micropython,toolmacher/micropython,henriknelson/micropython,hiway/micropython,ryannathans/micropython,blazewicz/micropython,tuc-osg/micropython,hiway/micropython,TDAbboud/micropython,ganshun666/micropython,infinnovation/micropython,dmazzella/micropython,chrisdearman/micropython,deshipu/micropython,adafruit/micropython,tuc-osg/micropython,MrSurly/micropython-esp32,Timmenem/micropython,dxxb/micropython,AriZuu/micropython,tralamazza/micropython,Peetz0r/micropython-esp32,lowRISC/micropython,torwag/micropython,PappaPeppar/micropython,infinnovation/micropython,cwyark/micropython,torwag/micropython,matthewelse/micropython,emfcamp/micropython,Peetz0r/micropython-esp32,alex-march/micropython,dxxb/micropython,dmazzella/micropython,oopy/micropython,dmazzella/micropython,TDAbboud/micropython,swegener/micropython,hosaka/micropython,toolmacher/micropython,MrSurly/micropython,MrSurly/micropython,alex-robbins/micropython,selste/micropython,MrSurly/micropython-esp32,hosaka/micropython,HenrikSolver/micropython,ganshun666/micropython,MrSurly/micropython,deshipu/micropython,AriZuu/micropython,Peetz0r/micropython-esp32,HenrikSolver/micropython,emfcamp/micropython,redbear/micropython,MrSurly/micropython-esp32,puuu/micropython,oopy/micropython,redbear/micropython,PappaPeppar/micropython,micropython/micropython-esp32,infinnovation/micropython,trezor/micropython,henriknelson/micropython,trezor/micropython,puuu/micropython,pfalcon/micropython,trezor/micropython,selste/micropython,pramasoul/micropython,blazewicz/micropython,adafruit/circuitpython,tobbad/micropython,matthewelse/micropython,mhoffma/micropython,pramasoul/micropython,redbear/micropython,tobbad/micropython,hosaka/micropython,oopy/micropython,AriZuu/micropython,alex-robbins/micropython,redbear/micropython,kerneltask/micropython,SHA2017-badge/micropython-esp32,micropython/micropython-esp32,SHA2017-badge/micropython-esp32,hiway/micropython,lowRISC/micropython,kerneltask/micropython,MrSurly/micropython-esp32,puuu/micropython,jmarcelino/pycom-micropython,cwyark/micropython,mhoffma/micropython,selste/micropython,alex-march/micropython,tuc-osg/micropython,AriZuu/micropython,pfalcon/micropython,redbear/micropython,lowRISC/micropython,pozetroninc/micropython,pramasoul/micropython,pfalcon/micropython,matthewelse/micropython,toolmacher/micropython,HenrikSolver/micropython,swegener/micropython,swegener/micropython,oopy/micropython,emfcamp/micropython,jmarcelino/pycom-micropython,turbinenreiter/micropython,selste/micropython,emfcamp/micropython,ryannathans/micropython,matthewelse/micropython,PappaPeppar/micropython,dxxb/micropython,infinnovation/micropython,swegener/micropython,swegener/micropython,pozetroninc/micropython,turbinenreiter/micropython,bvernoux/micropython,kerneltask/micropython,chrisdearman/micropython,TDAbboud/micropython,puuu/micropython,MrSurly/micropython-esp32,tobbad/micropython,matthewelse/micropython,toolmacher/micropython,micropython/micropython-esp32,adafruit/micropython,cwyark/micropython,TDAbboud/micropython,Peetz0r/micropython-esp32,cwyark/micropython,chrisdearman/micropython,ganshun666/micropython,tobbad/micropython,tralamazza/micropython,micropython/micropython-esp32,hosaka/micropython,dxxb/micropython,trezor/micropython,MrSurly/micropython,AriZuu/micropython,turbinenreiter/micropython,alex-robbins/micropython,bvernoux/micropython,SHA2017-badge/micropython-esp32,bvernoux/micropython,pfalcon/micropython,infinnovation/micropython,adafruit/circuitpython,ryannathans/micropython,MrSurly/micropython,torwag/micropython,tuc-osg/micropython,lowRISC/micropython,Peetz0r/micropython-esp32,deshipu/micropython,pozetroninc/micropython,alex-robbins/micropython,kerneltask/micropython,TDAbboud/micropython,turbinenreiter/micropython,adafruit/micropython