feat: Serialize bytes-based stored structs faster

[huitseeker]

Without specialization, Rust forces Serde to treat &[u8] just like any other slice and Vec just like any other vector (and iterates through elements). Serde can do better.

This uses [serde_with]'s features to opt into serialization optimizations originally brought about by serde_bytes on these bytes-based types we serialize often. Most specifically it uses serde_with's serde_as attribute.

See https://github.com/jonasbb/serde_with/pull/277/files

Perf: cargo test cert -- --nocapture in fastx_types
With this PR:

Write Cert: 61 microsec

Without:

Write Cert: 100 microsec

Perf: cargo run --release --bin bench -- --db-cpus=2 --tcp-connections=10
With this PR:

    Total time: 2586051us, items: 40000, tx/sec: 15467.599053537613

Without:

   Total time: 2759989us, items: 40000, tx/sec: 14492.811384393199

[gdanezis]

Oh I like that. Can we use this for the content of objects? This is likely to be the biggest byte string of them all.

[huitseeker]

With:

Write Cert: 52 microsec
Write Order: 223 microsec
Write Vote: 229 microsec

Without:

Write Cert: 93 microsec
Write Vote: 264 microsec
Write Order: 274 microsec

[sblackshear]

Both the object trick and the #'s associated with it LGTM! Thanks.