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/frame/head.rs

@@ -1,6 +1,6 @@
 use super::StreamId;
 
-use bytes::{BufMut, BigEndian};
+use bytes::{BigEndian, BufMut};
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub struct Head {