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
issue