Since TOML will not encode C-like enums as strings, we do it
ourselves. This is also worthwhile so that we can get the
lowercase "bitcoin" and "testnet" as encodings for the actual
enum values, which are more verbose and camel case.
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.
Andrew Poelstra