Support variable-length keys

[petar]

The trie can be updated to support variable length keys. Naturally, this requires updating the logic in all methods (Add, Find, Remove, etc.)

This functionality was already needed in libp2p/go-libp2p-asn-util#8, but only for Add and Find.
There, we opted to make a copy of the trie structure and update just Add and Find, due to time constraints.

We need to reconcile the two implementations and have just one (here) which supports variable-length keys on all of its methods.