Port to bincode 2.0

[gbin]

https://docs.rs/crate/bincode/latest

The traits are incompatible so a codebase with bincode 2 types cannot use rkv.

[badboy]

Note that we currently do the bare minimum to keep rkv working for our internal use.
We currently do not have resources to do anything more, especially incorporating breaking changes. Like if the bincode format changed in any way in v2 we need a migration path, and that's just not something we can currently fit in.

[gbin]

Makes sense @badboy and I suspected that, thanks.

I tried to go through it quickly but the change is larger than expected.