Switch to using `bytes` for serialization

[gkelly]

Description of changes: Simplify the serialization logic by switching to using the bytes crate instead of manually tracking buffer position and read/write sizes.

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[gkelly]

I know the advice here is to be frugal with the addition of new dependencies, but this is the Cargo.toml for bytes: https://github.com/tokio-rs/bytes/blob/master/Cargo.toml, it's a very thin crate and is designed for doing exactly the kind of thing being done manually here currently.