Pre-RFC: Stabilize `#[bench]`, `Bencher` and `black_box`

[nikomatsakis]

@tomaka

if you want to use a third party library you add a dependency to awesome_library and you use #[awesome_library_bench].

Or perhaps

use awesome_library::bench; // shadow the default bench macro 

#[bench]
fn foo() { ... }

though I've not thought that through at all =)