Use key Qunbound instead of hash value hash_unused for free entries

Use key Qunbound instead of hash value hash_unused for free entries

Previously, free hash table entries were indicated by both hash value
hash_unused and key Qunbound; we now rely on the latter only.
This allows us to change the hash representation to one that does not
have an unused value.

* src/lisp.h (hash_unused): Remove.
All uses adapted to calling hash_unused_entry_key_p on the key instead.
The hash values for unused hash table entries are now undefined; all
initialisation and assignment to hash_unused has been removed.