* 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.
Check for the `std` feature outside the macro body rather than inside,
so that we get the `std` feature in `cretonne-codegen`, rather than in
whatever crate the macro is being expanded in.
