MAINT: clean up some macros in scalarapi.c
MAINT: clean up some macros in scalarapi.c

Using a macro in an if without parens is just weird.

Also, simplies macros to use existing macros, and add comments to make clear why `Void` is special.

WarrenWeckesser/numpy,charris/numpy,WarrenWeckesser/numpy,mhvk/numpy,pdebuyl/numpy,jakirkham/numpy,endolith/numpy,mhvk/numpy,anntzer/numpy,WarrenWeckesser/numpy,madphysicist/numpy,anntzer/numpy,seberg/numpy,jakirkham/numpy,charris/numpy,madphysicist/numpy,abalkin/numpy,anntzer/numpy,madphysicist/numpy,pdebuyl/numpy,simongibbons/numpy,abalkin/numpy,numpy/numpy,jakirkham/numpy,pbrod/numpy,numpy/numpy,pbrod/numpy,mhvk/numpy,abalkin/numpy,endolith/numpy,pdebuyl/numpy,rgommers/numpy,numpy/numpy,seberg/numpy,charris/numpy,grlee77/numpy,endolith/numpy,pdebuyl/numpy,WarrenWeckesser/numpy,charris/numpy,pbrod/numpy,mattip/numpy,seberg/numpy,mhvk/numpy,WarrenWeckesser/numpy,grlee77/numpy,numpy/numpy,mattip/numpy,grlee77/numpy,mattip/numpy,pbrod/numpy,rgommers/numpy,simongibbons/numpy,madphysicist/numpy,pbrod/numpy,madphysicist/numpy,simongibbons/numpy,simongibbons/numpy,jakirkham/numpy,simongibbons/numpy,anntzer/numpy,mattip/numpy,rgommers/numpy,mhvk/numpy,grlee77/numpy,rgommers/numpy,endolith/numpy,grlee77/numpy,seberg/numpy,jakirkham/numpy