BlockHash Refactor

[DieracDelta]

Closes #10 . This PR adds the following functionality:

• Renames what was originally BlockHash to InternalHash and makes InternalHash private.
• Adds five public wrapper types around InternalHash: BlockHash (hash of implementors of BlockContents), LeafHash (hash of Leaf), TransactionHash (hash of BlockContents::Transaction), VerifyHash (special case used to verify QuorumCertificates), StateHash (hash of implementors of State). This is done via the macro gen_hash_wrapper_type since I couldn't seem to think of an easier way to sidestep boilerplate code without exposing the inner InternalHash type.
• Uses serde_bytes to hash the underlying array during serialization. Note that serde_bytes does not seem to support const generics, so I directly copied in the wrapper given in that issue.
• Adds a Debug implementation to InternalHash and its wrapper types.

@nmccarty if this is too large, I'm happy to split this in several smaller PRs.

I'm also uncertain if the types I've added make sense. This spot in particular was causing me a bit of confusion. It seems like in extends_from, we track the leaf hashes up the tree. So parent is a LeafHash. And we compare parent to node, which makes node also a LeafHash. I changed the extends_from invocation to reflect that by passing in locked_qc.leaf_hash instead of locked_qc.block_hash. The tests still pass after this change.