chore: configure rustfmt

[louib]

I've been wanting to do that for a while. The default max_width for rustfmt is 100, which results in many split lines, given the verbosity of Rust. The PR should not be too difficult to review, but you don't have to trust me. You can copy the rustfmt.toml file and run the following script, and you should get to the same result:

#!/usr/bin/env bash
files=$(find . -name '*.rs')
IFS=$'\n'; for file in $files; do
    rustfmt "$file"
done

[droidmonkey]

We use 120 in our clang format, I'm all for longer lines

[louib]

@droidmonkey yeah I don't mind longer lines either, we could definitely consider bumping to 120.