Use rustfmt to format the source code

[Detegr]

Rustfmt automatically formats Rust code to conform Rust style guide. The current code base of fd is hand formatted and I think it would be easier for contributors if we formatted the code with rustfmt.

What do you think? If there's some irritating style decisions in rustfmt, it is possible to tweak it to some extent.

[Detegr]

For a reference, I pushed a commit to my fork which formats the repo using rustfmt with default settings.

[sharkdp]

Sounds good!

Should we also enforce this via Travis/Appveyor in the future? Can rustfmt be used as a checker?

[Detegr]

Yes, it should be possible. I'll look into that.