rust: max_width=100, other rustfmt settings. #27071
These are the 12 stable and documented configuration options,
set to their default values.
use_small_heuristics is only stabilized in rustfmt 0.9, so maintain
support for 0.8.x for now by commenting it out.
comment_width is unstable and did nothing, since wrap_comments defaults
to false.
Default values gotten from `rustfmt --print-config default rustfmt.toml`.
https://github.com/rust-lang-nursery/rustfmt/blob/e7932fa9c2591c45a37a24305de90cb63128afcf/Configurations.md