Cleanup also hashtable.c.
Rename _Py_hashtable_t members:

* Rename entries to nentries
* Rename num_buckets to nbuckets

Move PyInterpreterState.fs_codec into a new
PyInterpreterState.unicode structure.

Give a name to the fs_codec structure and use this structure in
unicodeobject.c.

Remove the _PyUnicode_ClearStaticStrings() function from the C API.
Make the function fully private (declare it with "static").

_Py_hashtable_t values become regular "void *" pointers.

* Add _Py_hashtable_entry_t.data member
* Remove _Py_hashtable_t.data_size member
* Remove _Py_hashtable_t.get_func member. It is no longer needed
  to specialize _Py_hashtable_get() for a specific value size, since
  all entries now have the same size (void*).
* Remove the following macros:

  * _Py_HASHTABLE_GET()
  * _Py_HASHTABLE_SET()
  * _Py_HASHTABLE_SET_NODATA()
  * _Py_HASHTABLE_POP()

* Rename _Py_hashtable_pop() to _Py_hashtable_steal()
* _Py_hashtable_foreach() callback now gets key and value rather than
  entry.
* Remove _Py_hashtable_value_destroy_func type. value_destroy_func
  callback now only has a single parameter: data (void*).

Rewrite _tracemalloc to store "trace_t*" rather than directly
"trace_t" in traces hash tables. Traces are now allocated on the heap
memory, outside the hash table.

Add tracemalloc_copy_traces() and tracemalloc_copy_domains() helper
functions.

Remove _Py_hashtable_copy() function since there is no API to copy a
key or a value.

Remove also _Py_hashtable_delete() function which was commented.

Add key_destroy_func and value_destroy_func parameters to
_Py_hashtable_new_full().

marshal.c and _tracemalloc.c use these destroy functions.

Rewrite _Py_hashtable_t type to always store the key as
a "const void *" pointer. Add an explicit "key" member to
_Py_hashtable_entry_t.

Remove _Py_hashtable_t.key_size member.

hash and compare functions drop their hash table parameter, and their
'key' parameter type becomes "const void *".

Rewrite how the _tracemalloc module stores traces of other domains.
Rather than storing the domain inside the key, it now uses a new hash
table with the domain as the key, and the data is a per-domain traces
hash table.

* Add tracemalloc_domain hash table.
* Remove _Py_tracemalloc_config.use_domain.
* Remove pointer_t and related functions.

Add a new _Py_HashPointerRaw() function which avoids replacing -1
with -2 to micro-optimize hash table using pointer keys: using
_Py_hashtable_hash_ptr() hash function.

… now that we allow subtypes of PyCFunction.

Also add PyCMethod_CheckExact() and PyCMethod_Check() for checks against the PyCMethod subtype.