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/build.rs

@@ -67,9 +67,7 @@ fn main() {
         .arg("--out-dir")
         .arg(out_dir)
         .status()
-        .expect(
-            "Failed to launch second-level build script; is python installed?",
-        );
+        .expect("Failed to launch second-level build script; is python installed?");
     if !status.success() {
         process::exit(status.code().unwrap());
     }
@@ -132,16 +130,14 @@ impl Isa {
 /// Returns isa targets to configure conditional compilation.
 fn isa_targets(cretonne_targets: Option<&str>, target_triple: &str) -> Result<Vec<Isa>, String> {
     match cretonne_targets {
-        Some("native") => {
-            Isa::from_arch(target_triple.split('-').next().unwrap())
-                .map(|isa| vec![isa])
-                .ok_or_else(|| {
-                    format!(
-                        "no supported isa found for target triple `{}`",
-                        target_triple
-                    )
-                })
-        }
+        Some("native") => Isa::from_arch(target_triple.split('-').next().unwrap())
+            .map(|isa| vec![isa])
+            .ok_or_else(|| {
+                format!(
+                    "no supported isa found for target triple `{}`",
+                    target_triple
+                )
+            }),
         Some(targets) => {
             let unknown_isa_targets = targets
                 .split(',')