Integrate with rustfmt

[IanConnolly]

I took #49 and ran with it a bit as I want to get this over the line.

Differences:

• New RustFmt command
• We fill the location-list rather than the quickfix-list, due to location-list and RustFmt itself being buffer-local.
• No more echo'ing of unparseable errors.
• Documentation.
• Misc code cleanup; safety.

Will also fix #43.

Thoughts @steveklabnik, @grncdr?

[rust-highfive]

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[pittma]

Hi. I'm trying this out, and it works great!

Just a quick note: here you have rust_fmt_autosave as the bool to check for autosave, but the lookup uses g:rustfmt_autosave (note the extra underscore).

[IanConnolly]

Good catch! Missed that :) Will fix this evening.

[steveklabnik]

This looks pretty great! Since @grncdr 's commits are still in there, I'd like their comments before merging. Seems like this is a good way to go, but want to make sure :)

[IanConnolly]

Fixed said typo up.

Ready whenever you are @steveklabnik @grncdr :)

[grncdr]

I have no preference for location list or quickfix. This looks great to me, nice work @IanConnolly 👍

[steveklabnik]

Okay! Let's do it! Thank you both for all the work.