Remove `fn deserialize` and instead provide a trait `fn decode_exact`
for testing.
Refactor errors, so that we can now cleanly distinguish between
`Error::UnexpectedEnd` which means that decoding might work given
more data, and `Error::Invalid`, which means that no amount of
additional data will decode.
While at it, cleanup the variants of `Error::Invalid`, and provide `ControlType`.
a568e7f484