Implement avl find

Solves #57 

**Note:** We return a `struct avl_el*` here, so we do not search for an actual _element_ but a _node_ where the element we look for should be in.
I don't see how you can ever get an `iter` which is not `NULL` but which has a different hash for what you're looking for.
Hence, the entire if may be omitted.