(struct charset): Remove dependency on hash-table internals

(struct charset): Remove dependency on hash-table internals

`struct charset` kept an index into the internal `key_and_value` array
of hash tables, which only worked because of details of how
hash-tables are handled.  Replace it with a reference to the
value stored at that location in the hash-table, which saves us an
indirection while at it.

* src/charset.h (struct charset): Replace `hash_index` field with
`attributes` field.
(CHARSET_ATTRIBUTES): Simplify accordingly.
(CHARSET_HASH_INDEX): Delete unused macro.
* src/charset.c (Fdefine_charset_internal):
* src/pdumper.c (dump_charset): Adjust accordingly.
(dump_charset_table): Set the referrer since that's needed while
dumping Lisp_Object fields.

(cherry picked from commit 33b8d5b6c5a22bab069cdac4bddda932b3d18b13)