Abstract predicate and constant for unused hash keys

Abstract predicate and constant for unused hash keys

Qunbound is used for many things; using a predicate and constant for
the specific purpose of unused hash entry keys allows us to locate
them and make changes much more easily.

* src/lisp.h (HASH_UNUSED_ENTRY_KEY, hash_unused_entry_key_p):
New constant and function.
* src/comp.c (compile_function, Fcomp__compile_ctxt_to_file):
* src/composite.c (composition_gstring_cache_clear_font):
* src/emacs-module.c (module_global_reference_p):
* src/fns.c (make_hash_table, maybe_resize_hash_table, hash_put)
(hash_remove_from_table, hash_clear, sweep_weak_table, Fmaphash):
* src/json.c (lisp_to_json_nonscalar_1):
* src/minibuf.c (Ftry_completion, Fall_completions, Ftest_completion):
* src/print.c (print, print_object):
Use them.