Represent hash table weakness as an enum internally

Represent hash table weakness as an enum internally

This takes less space (saves an entire word) and is more type-safe.
No change in behaviour.

* src/lisp.h (hash_table_weakness_t): New.
(struct Lisp_Hash_Table): Replace Lisp object `weak` with enum
`weakness`.
* src/fns.c
(keep_entry_p, hash_table_weakness_symbol): New.
(make_hash_table): Retype argument.  All callers updated.
(sweep_weak_table, Fmake_hash_table, Fhash_table_weakness):
* src/alloc.c (purecopy_hash_table, purecopy, process_mark_stack):
* src/pdumper.c (dump_hash_table):
* src/print.c (print_object): Use retyped field.