_PyMalloc_{Malloc, Realloc}: Strive to meet the doc's promises about

_PyMalloc_{Malloc, Realloc}:  Strive to meet the doc's promises about
what these do given a 0 size argument.  This is so that when pymalloc
is enabled, we don't need to wrap pymalloc calls in goofy little
routines special-casing 0.  Note that it's virtually impossible to meet
the doc's promise that malloc(0) will never return NULL; this makes a
best effort, but not an insane effort.  The code does promise that
realloc(not-NULL, 0) will never return NULL (malloc(0) is much harder).

_PyMalloc_Realloc:  Changed to take over all requests for 0 bytes, and
rearranged to be a little quicker in expected cases.

All over the place:  when resorting to the platform allocator, call
free/malloc/realloc directly, without indirecting thru macros.  This
should avoid needing a nightmarish pile of #ifdef-ery if PYMALLOC_DEBUG
is changed so that pymalloc takes over all Py(Mem, Object} memory
operations (which would add useful debugging info to PyMem_xyz
allocations too).