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.
I think this is what I want to do for everything json-visible...perhaps
I will not be able to keep the macro for it though, since there are
some clever variations on it (e.g. blocks should have their header's
hash as a field, txes should appear as txids unless vebose output is
requested, etc.)
Andrew Poelstra