core: add min and max to cmp, re-export various places.
core: add abs to num.
std: add stats.
rustc and std: teach about #[bench], modernize to use quote_expr! some.
auto merge of #4515 : graydon/rust/bench-harness, r=graydon

This is scaffolding for the new #[bench] attribute for marking unit tests as benchmarks. They are run with the --bench flag that the test runner now accepts. The runner automatically calibrates a test loop to an appropriate count to get a good per-iteration measurement.