* 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.
* Use FxHashMap instead of HashMap for better performance
* Replace the binary search in the coalescing pass with a FxHashMap
This speeds up coalescing by up to 16% and overall compilation by 9%
The dbg! macro expands to nothing in no_std mode, so variables that are
only used for debugging prompt unused variable warnings.
Also, allow unstable_features in no_std builds, since they use
feature(alloc), which is an unstable feature.
