Represents combinations of flags with a '+' separated string of nicks,
or an empty string for no flags set.
Note that most flag types will ignore any flags using multiple bits when
serializing, since in most cases these flags cover all used bits.
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.
