Widespread, but mostly in _PyMalloc_Malloc: optimize away all expensive

Widespread, but mostly in _PyMalloc_Malloc:  optimize away all expensive
runtime multiplications and divisions, via the scheme developed with
Vladimir Marangozov on Python-Dev.  The pool_header struct loses its
capacity member, but gains nextoffset and maxnextoffset members; this
still leaves it at 32 bytes on a 32-bit box (it has to be padded to a
multiple of 8 bytes).