This is a massive simplification, fixes a couple endianness bugs (though
not all of them I don't think), should give a speedup, gets rid of the
`serialize_iter` crap.
We were conflicting with the Rust stdlib trait Hash, which is used
by various datastructures which need a general hash. Also implement
Hash for Sha256dHash so that we can use bitcoin hashes as keys for
such data structures.
Andrew Poelstra