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`struct composition` 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
key stored at that location in the hash-table, which saves us an
indirection while at it.

* src/composite.h (struct composition): Replace `hash_index` with
the actual `key`.
(COMPOSITION_KEY): Simplify accordingly.
(mark_composite): Declare.
* src/composite.c (get_composition_id): Adjust accordingly.
(mark_composite): New function.

* src/charset.c (mark_charset): Uncomment.
* src/lisp.h (mark_charset): Declare.
* src/alloc.c (garbage_collect): Call `mark_charset` and `mark_composite`.
* src/pdumper.c (hash_table_contents): Remove invalid comment, since
compositions aren't dumped.