hash: minor cleanups * lib/hash.h (hash_entry): Make opaque, by moving... * lib/hash.c (hash_entry): ...here. (hash_insert): Clarify restrictions on what can be inserted. (hash_get_next): Clarify when it is safe to remove an element during traversal. (check_tuning): Skip verification when tuning is known safe. (hash_initialize): Clarify restrictions on tuning. Signed-off-by: Eric Blake <ebb9@byu.net>