rt: Fix child-iteration bug in crate map.
std: add #[cfg(test)] reference to extra so we can benchmark libstd.
std: add preliminary str benchmark.
std: add #[bench] benchmarks for global and local heaps.
std: add #[bench] benchmarks for rand.
std: add #[bench] benchmarks for num::strconv
std: add benchmark for allocating-and-dropping a struct with a dtor.
std: various additional language benchmarks in util.
auto merge of #7980 : graydon/rust/misc-benchmarks, r=catamorphism

Some machinery for enabling #[bench] benchmarks in std and some examples showing how to write them.