Bincode is unmaintained

Bincode has recently become unmaintained (see https://crates.io/crates/bincode).

The project lists possible successors as

- [wincode](https://crates.io/crates/wincode)
- [postcard](https://crates.io/crates/postcard)
- [rkyv](https://crates.io/crates/rkyv)

I imagine it would make sense to switch to one of those.

There is already an issue about using rkyv (https://github.com/google/tarpc/issues/437), but it looks like switching to it might be a bigger lift. Still, it might be sensible long-term, depending on how @tikue and others feel.

I have some experience using rkyv, and Postcard doesn't look too difficult (I have no experience with wincode). @tikue I would be happy to help contribute to the effort, if you are open to it.

*edit:* I realised there is the added complexity of [tokio-serde](https://github.com/carllerche/tokio-serde) for the `serde-transport` feature.