hash: minor cleanups

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>