Clarify empty match arm format

[varkor]

Prefer pattern => {} over pattern => ().

Fixes #146.

[calebcartwright]

I'm really surprised there hasn't been more feedback on this one way or the other. I'm going to continue to hold off on merging because doing so would for the first time put us in a tricky situation where rustfmt would either have to deliberately introduce breaking changes or would conflict with/fail to follow the style guide.

If we get an opportunity to push through some breaking changes (and there's a few that desperately need to be made) will be sure that this is accounted for, likely with an accompanying config option in rustfmt that allows users to use whichever they want

[calebcartwright]

Closing as the style guide no longer lives in this repo (it's in r-l/rust), and we'll use #146 as the tracking issue to discuss style plans