bpo-40602: Rename hashtable.h to pycore_hashtable.h (GH-20044)

* Move Modules/hashtable.h to Include/internal/pycore_hashtable.h
* Move Modules/hashtable.c to Python/hashtable.c
* Python is now linked to hashtable.c. _tracemalloc is no longer
  linked to hashtable.c. Previously, marshal.c got hashtable.c via
  _tracemalloc.c which is built as a builtin module.