Update to rustfmt-preview (#348)

* Update to rustfmt-preview.

* Run "cargo fmt --all" with rustfmt 0.4.1.

rustfmt 0.4.1 is the latest release of rustfmt-preview available on the
stable channel.

* Fix a long line that rustfmt 0.4.1 can't handle.

* Remove unneeded commas left behind by rustfmt.

lib/codegen/src/result.rs

@@ -12,10 +12,7 @@ pub enum CtonError {
     /// This always represents a bug, either in the code that generated IR for Cretonne, or a bug
     /// in Cretonne itself.
     #[fail(display = "Verifier error: {}", _0)]
-    Verifier(
-        #[cause]
-        verifier::Error
-    ),
+    Verifier(#[cause] verifier::Error),
 
     /// An implementation limit was exceeded.
     ///