add `cargofmt` to clean up cargo.toml files

[gnunicorn]

When editing cargo.toml files a few things happen that make the result a little unclean:

• when updating the version, we sometimes have to add it and that leads to unclean spacing in inline-tables
• the order of dependency attributes is mixed up, instead it should be fixed and deterministic – to allow for quicker parsing. We should think a bit about a good order, alphabetical might not be the best suited but something like : version, optional, default-features, features = , git, path ...

A new subcommand should enforce this formatting, potentially have a checking function that can be invoked as either "warn" or "error" on cargo unleash check.

[gilescope]

One day it will come as standard ... rust-lang/rustfmt#4091