Don't dump the `hash` vector if it will need to be recomputed anyway

Don't dump the `hash` vector if it will need to be recomputed anyway

* src/fns.c (hash_table_rehash): Only set `hash` field at the end.
(sweep_weak_table): Only set slot of `hash` vector when that vector exists.
(Fhash_table_count): No need to hash_rehash_if_needed any more.

* src/lisp.h (hash_rehash_needed_p): Test the presence of `hash` instead.

* src/pdumper.c (check_hash_table_rehash, dump_hash_table):
Set `hash` to nil to indicate that the table needs to be rehashed.