Make get_internal return a proper leaf object, and rdx_find the value corresponding to the key. If no such value is found, NULL is returned.
Make get_internal return a proper leaf object, and rdx_find the
value corresponding to the key. If no such value is found, NULL
is returned.

The goal is to avoid exposing any of the internals of the node
and leaf structures in the public radix tree API.

Also, remove useless key length parameter that was being passed
around the internal API.

j0sh/radixtree