Rearranged and added comments to object.h, to clarify many things

Rearranged and added comments to object.h, to clarify many things
that have taken me "too long" to reverse-engineer over the years.
Vastly reduced the nesting level and redundancy of #ifdef-ery.
Took a light stab at repairing comments that are no longer true.

sys_gettotalrefcount():  Changed to enable under Py_REF_DEBUG.
It was enabled under Py_TRACE_REFS, which was much heavier than
necessary.  sys.gettotalrefcount() is now available in a
Py_REF_DEBUG-only build.