[WIP] Fix ht interface

Resubmit from #105 onto master
So you would like me to return a status and not the hash. Okay. So when the user inserts a value, her/his provided hash function is used for the hashtable, but the user will not get the hash, because  he can recreate it easily, am I right?

We should _definitely_ state in the docs that the hash is not supposed to be a hash in the known form.