Merkle trees in Rust

The nodes are abstract, i.e. leaf and node. The hash is abstract. The tree storage is abstract Write the sparse merkle tree as a macro to generate trees of any power of 2 arity. Domain separation while hashing leaves and nodes.

1. Vanilla (inefficient) sparse merkle tree
2. Sparse merkle tree with optimizations from V. Buterin
3. Ethereum's Merkle Patricia trie
4. Compact merkle tree as described by Google's certificate transparency.

TODO

1. Make each tree usable as a feature.