gc: More verbose debugging

Add more DEBUG_printf statements to trace gc behaviour
gc: Keep debug statements at beginning of scope where possible
gc: Turn off debugging info again
Add missing “assert.h” file header inclusion from “nlr.h”
Prefix ARRAY_SIZE with micropython prefix MP_
Merge pull request #707 from eblot/master-v1.1.1-build-fixes

Fix missing declaration of assert()
Replace ARRAY_SIZE with MP_ARRAY_SIZE
Merge pull request #697 from stinos/gc-debug

gc: More verbose debugging
py: Separate MICROPY_PY_BUILTINS_COMPLEX from MICROPY_PY_BUILTINS_FLOAT.

One thing is wanting to do 1 / 2 and get something else but 0, and quite
another - doing rocket science ;-).
tests/float/: Skip tests if "math" module is not available.