Support range proofs

Nice work on this repo, seems like the merklized state storage backend is the transaction processing bottleneck in pretty much every blockchain project.

I'm thinking of replacing my own LevelDB-based tree implementation in my module [merk](https://github.com/mappum/merk) with urkel, although it would be nice if it supported generating/verifying range proofs (e.g. you query with a start key and end key, and get a proof of all the keys inclusively in the range (or before or after the range if the start or end keys aren't an exact match)).

From there, it would be pretty easy to implement a wrapper on top of urkel which implements the leveldown interface, but that's another story.

The issue requests range-proof support for the urkel tree, including proofs for keys inclusively between start and end keys and for ranges whose bounds are not exact matches. Begin by locating the existing proof-generation and verification entry points, then define completion as generating and verifying complete range proofs with both exact and non-exact bounds.

Written by the indexing model from the issue text.