Support code for rustc's built in unit-test and micro-benchmarking
framework.
Almost all user code will only be interested in `Bencher` and
`black_box`. All other interactions (such as writing tests and
benchmarks themselves) should be done via the `#[test]` and
`#[bench]` attributes.
See the [Testing Chapter](../book/ch11-00-testing.html) of the book for more details.