Because it's hot and the number of entries can reach the 1000s, so
linear insertion and search is bad.
This reduces runtime for `sqlite` and `UE4Game-HTML5-Shipping` by 3-4%,
and a couple of other benchmarks (`sqlite`, `godot`, `clang`) by smaller
amounts.
It also increases runtime for `mono` and `tanks` by about 1%; this seems
to be due to incidental changes in which functions are inlined more than
algorithmic changes.
* 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.
