This improves build times as it allows cargo to reuse more previous
build results when building crates separately.
Also remove the serde-pickle dependency. It was only used in a single
test and caused pulling in of quite a few dependencies.
Serialization and deserialization are asymetrical: ser uses string literals
and de uses a dedicated `struct`, leading to possible inconsistencies.
The idea is to rely on a single `struct` for both ser and de. The only
problem is that, in order to avoid unneeded copies, `buffer` requires an owned
`ByteBuf` for de and a borrowed `Bytes` for ser. `Either` could help adapting
to both situations.
