What made the issue fixed in the parent commit harder to spot
for me was the multitude of concepts and terms involved in
encoding: What's *serializing* vs. *encoding*? Why is there
this odd `Frame::to_bytes`?
So I decided to clean up a bit. I removed `fn serialize` so that the
logic stays close to the data, and instead of it provide
`fn encode_vec` in `trait Encode` so that we don't add up with
one-offs like `Frame::to_bytes` again.
Also we make use of `encode_vec` in the tests.
cbd2a7070c