[WIP] Fix ht interface

Resubmit from #105 onto master
The hash is nowhere sufficient. The user may use a key of some soft, used by the hash function, to retrieve elements: the user may create a dummy object (a "comparable") and feed it to `ht_find`. That function will then search an object which will compare to the one provided and return that one to the user.
Also, the hash may be generated by the user anyways, since _he_ provides the hash function in the first place.