bpo-40602: Optimize _Py_hashtable for pointer keys (GH-20051)

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.