Replace unmaintained "bincode" serialization/deserialization dependency with "postcard"

[ocean]

The bincode library used for serializing and deserializing has been marked as unmaintained (RUSTSEC-2025-0141).

The maintainer makes several replacement recommendations as do RUSTSEC:

• wincode - this is quite recent

• postcard - maintainer has many Rust libraries they maintain, lots of activity, appears stable, drop-in replacement, 60+ contributors, used by 7,000+ other repos

• bitcode - seems fine, but lower activity and not many maintainers

• rkyv - looks good, but would require many more changes across the codebase to implement

I've done up a PR to replace bincode with postcard, enjoy! 😁

#2208

[ocean]

postcard serialization output is not the same as bincode, so this has broken compatibility with the go-libsql library and possibly other bindings.

wincode is apparently compatible directly with bincode so investigating that now.