Alter AllocSet routines so that requests larger than

Alter AllocSet routines so that requests larger than
ALLOC_BIGCHUNK_LIMIT are always allocated as separate malloc() blocks,
and are free()d immediately upon pfree().  Also, if such a chunk is enlarged
with repalloc(), translate the operation into a realloc() so as to
minimize memory usage.  Of course, these large chunks still get freed
automatically if the alloc set is reset.
I have set ALLOC_BIGCHUNK_LIMIT at 64K for now, but perhaps another
size would be better?