* src/fns.c: Use `EQ (key, Qunbound)` to check if a slot is in use

* src/fns.c: Use `EQ (key, Qunbound)` to check if a slot is in use

(make_hash_table): Use Qunbound for the key_and_value table.
(maybe_resize_hash_table): Set new key_and_value slots to Qunbound.
(hash_table_rehash): Don't bother copying the old table of hashes since
we're recomputing it completely.
(hash_table_rehash): Use hash_rehash_needed_p more.
(hash_put): Add assertion that the slot was indeed considered empty.
(hash_remove_from_table, hash_clear, sweep_weak_table): Set empty
slot's key to Qunbound.
(Fmaphash): Use `EQ (key, Qunbound)` to check if a slot is in use.

* src/lisp.h (struct Lisp_Hash_Table): Update comments.