add canonical rustfmt

[TheButlah]

Its nice to have a vendored rustfmt.toml. because when people use settings that differ from the defaults, the rustfmt.toml will take precedence.

[BiagioFesta]

I use a specific rustfmt.toml locally, I'd like to vendor it, but many useful option are nightly at the moment.

My local:

imports_granularity = "Item"
format_code_in_doc_comments = true

and that's why I ignored it for now.

• It is good to vendor a +nightly configuration?
• I'd prefer to vendor with filename .rustfmt.toml (hidden file); that should help keeping root directly a little bit "cleaner"

[BiagioFesta]

probably better .rustfmt.toml

[TheButlah]

It is good to vendor a +nightly configuration?

I know some people do this, but since it eliminates the ability to use standard toolchain pinning via rust-toolchain (if you also want to build with stable), I don't personally advocate for nightly rustfmt.

[MOZGIII]

I think I know a good setup for this:

• set rust-toolchain.toml to use nightly
• vendor the nightly rustfmt settings
• at CI run tests and clippy against stable and the rest targets
• check rustfmt against nightly (version specified via the toml)