Stop using max_width=80 for rustfmt

We have two options:

• use max_width=79 to match our C coding standards
• use the default max_width=100 to match the Rust defaults and avoid wrapping bugs in rustfmt

For details, see: https://trac.torproject.org/projects/tor/ticket/26972#comment:2 https://trac.torproject.org/projects/tor/ticket/26972#comment:3