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/module/src/backend.rs

@@ -2,8 +2,8 @@
 
 use DataContext;
 use Linkage;
-use ModuleNamespace;
 use ModuleError;
+use ModuleNamespace;
 use cretonne_codegen::Context;
 use cretonne_codegen::isa::TargetIsa;
 use cretonne_codegen::{binemit, ir};