Adding Merkle trees

- First cut at a Merkle tree representation in code.
--- Unofficially, a Merkle tree might not be valid with respect to
itself or its included hashes, which is why there's a ByteTwiddling
extension method to validate a given node.
- Two initial copy-paste implementations, one of which is cheating
because I'm "not there yet" with respect to proper block chain
construction and validation.
--- I think I forced my own hand here, because I have functioning code
that wouldn't otherwise compile without doing this change.