bpo-40609: Remove _Py_hashtable_t.key_size (GH-20060)

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 *".