Add new string hash table based on htab_t.

	* hash.h (struct string_tuple): New.
	(hash_string_tuple): Likewise.
	(eq_string_tuple): Likewise.
	(string_tuple_alloc): Likewise.
	(str_hash_find): Likewise.
	(str_hash_find_n): Likewise.
	(str_hash_delete): Likewise.
	(str_hash_insert): Likewise.
	(str_htab_create): Likewise.