Inlined and specialised hash table look-up

Inlined and specialised hash table look-up

This improves performance in several ways.  Separate functions are
used depending on whether the caller has a hash value computed or not.

* src/fns.c (hash_lookup_with_hash, hash_lookup_get_hash): New.
(hash_lookup): Remove hash return argument.
All callers adapted.

hash_lookup_with_hash hash_hash_t arg