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