Use rustfmt to enforce consistent formatting

This change adds a .rustfmt.toml that includes ALL supported settings,
12 of which we have overridden to attempt to cater to our own
proclivities.

rustfmt is checked in the rust-nightly CI job.

src/proto/error.rs

@@ -1,5 +1,5 @@
-use frame::Reason;
 use codec::RecvError;
+use frame::Reason;
 
 use std::io;