_PyObject_DebugRealloc(): rewritten to let the underlying realloc do

most of the work.  In particular, if the underlying realloc is able to
grow the memory block in place, great (this routine used to do a fresh
malloc + memcpy every time a block grew).  BTW, I'm not so keen here on
avoiding possible quadratic-time realloc patterns as I am on making
the debug pymalloc more invisible (the more it uses memory "just like"
the underlying allocator, the better the chance that a suspected memory
corruption bug won't vanish when the debug malloc is turned on).