Replace bincode with postcard (or vice versa)

We have two serializer libraries that do pretty much the same (fast no-std-ready serialization): [bincode](https://crates.io/crates/bincode) and [postcard](https://crates.io/crates/postcard).
Most things in LibAFL use postcard right now, except for concolic.

We should remove bincode - unless it's more performant, then we should move everything to it and remove postcard.