Python/mashal.c uses Modules/hashtable.h.

Python/mashal.c indirectly gets Modules/hashtable.c implementation via Modules/_tracemalloc.c which is built as a builtin module.

I propose to make the "hashtable" more standard:

* Move Modules/hashtable.h to Include/internal/pycore_hashtable.h
* Move Modules/hashtable.c to Python/hashtable.c

Attached PR implements this change but also changes the default memory allocator to PyMem_Malloc/PyMem_Free which is faster than PyMem_RawMalloc/PyMem_RawFree (current default) for memory blocks <= 512 bytes.

It remains an internal C API which cannot be used outside CPython.