[Rust] code style: restrict line width to 90 characters?

[Chao Sun]

Hi Rust developers,

Just want to know if anyone like the idea to restrict the line width to 90
characters for Rust, similar to the C++ coding style. Personally I found it
helpful when you need to keep multiple windows in a monitor. This can
easily be enforced via rustfmt. If there's no objection, I can open a JIRA
for this and apply the change to the existing codebase.

Thanks,
Chao