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.

_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.

Optimize _Py_hashtable_get() and _Py_hashtable_get_entry() for
pointer keys:

* key_size == sizeof(void*)
* hash_func == _Py_hashtable_hash_ptr
* compare_func == _Py_hashtable_compare_direct

Changes:

* Add get_func and get_entry_func members to _Py_hashtable_t
* Convert _Py_hashtable_get() and _Py_hashtable_get_entry() functions
  to static nline functions.
* Add specialized get and get entry for pointer keys.

* 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.

In the experimental isolated subinterpreters build mode, the GIL is
now per-interpreter.

Move gil from _PyRuntimeState.ceval to PyInterpreterState.ceval.

new_interpreter() always get the config from the main interpreter.