Add rust-version to each Cargo.toml

[thomcc]

rust-version in Cargo.toml support landed in 1.56, so we should consider putting it in all of our Cargo.toml. I think 1.56 is also the version we support, so it would look like a rust-version = "1.56.0" line in them.

(If we actually support an older version, it doesn't break, but they get a warning)

[totsteps]

@thomcc can you assign this to me as well?