fix up how we invoke benchmarks

[kivikakk]

Don't make benchmarks a "feature" of a regular test run; put the benchmark in the benches directory so it's invoked with cargo bench.

(Awkwardly, note that cargo bench won't actually work on stable either—use cargo +nightly bench or similar.)

Fixes #151.
/cc @alerque

[alerque]

This seems like a reasonable improvement at least until the #[bench] attribute hits stable.

[kivikakk]

Great. Will merge.