Make cretonne-codegen's `result` module private, and instead just export
`CodegenError` and `CodegenResult` at the top level of the
cretonne-codegen crate. This makes them more consistent with Result and
Error types in other cretonne crates.
* 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.