Fix crash if user test munges hash table
* src/fns.c (restore_mutability) (hash_table_user_defined_call): New functions. (cmpfn_user_defined, hashfn_user_defined): Use them. (make_hash_table, copy_hash_table): Mark new hash table as mutable. (check_mutable_hash_table): New function. (Fclrhash, Fputhash, Fremhash): Use it instead of CHECK_IMPURE. * src/lisp.h (struct hash_table_test): User-defined functions now take pointers to struct Lisp_Hash_Table, not to struct hash_table_test. All uses changed. (struct Lisp_Hash_Table): New member ‘mutable’. * src/pdumper.c (dump_hash_table): Copy it. * test/src/fns-tests.el (test-hash-function-that-mutates-hash-table): New test, which tests for the bug.