This makes the merkle_tree module properly generic over the tree hash.
It still hard-codes a depth 32 tree, because Rust doesn't yet support
generic sizes, and we are unlikely to need to alter the tree depth in
future circuit changes.
The primary reason for migrating is that these crates provide APIs for
setting the personalisation string. This enables us to depend solely on
published crates, and thus publish our own crates.
The SIMD implementations are ported from libsodium.
Closes#67.
