Only dump the actual data, and the test encoded as an enum. This
simplifies dumping, makes dump files smaller and saves space at run
time.
* src/lisp.h (hash_table_std_test_t): New enum.
(struct Lisp_Hash_Table): Add frozen_test member, consuming no extra space.
* src/fns.c (hashfn_user_defined): Now static.
(hash_table_test_from_std): New.
(hash_table_rehash): Rename to...
(hash_table_thaw): ...this and rewrite.
* src/pdumper.c (hash_table_contents): Only include actual data, not
unused space.
(hash_table_std_test): New.
(hash_table_freeze): Set frozen_test from test.
(dump_hash_table): Dump frozen_test, not the whole test struct.
Don't bother other dumping fields that can be derived.