Store hash values as integers instead of Lisp_Object

Store hash values as integers instead of Lisp_Object

This improves typing, saves pointless tagging and untagging, and
prepares for further changes. The new typedef hash_hash_t is an alias
for EMACS_UINT, and hash values are still limited to the fixnum range.
We now use hash_unused instead of Qnil to mark unused entries.

* src/lisp.h (hash_hash_t): New typedef for EMACS_UINT.
(hash_unused): New constant.
(struct hash_table_test): `hashfn` now returns
hash_hash_t.  All callers and implementations changed.
(struct Lisp_Hash_Table): Retype hash vector to an array of
hash_hash_t.  All code using it changed accordingly.
(HASH_HASH, hash_from_key):
* src/fns.c (set_hash_index_slot, hash_index_index)
(hash_lookup_with_hash, hash_lookup_get_hash, hash_put):
(hash_lookup, hash_put): Retype hash value arguments
and return values.  All callers adapted.