Port benchmarks to criterion

[vks]

This uses more solid statistics and works on stable.
A new benchmark with random bits has been added.

[bluss]

Cool. I don't think a port to criterion is worthwhile. Look at that swarm of deps it brings in (see travis).

[vks]

Yes, but they are dev-dependencies. Anyway, it was useful for #18.

[vks]

Criterion requires Rust 1.23, while fixedbitset works with 1.12. But the old benchmarks shouldn't work on any stable, so I'm a bit confused. I could make the criterion benchmarks a non-default feature, what do you think?

[bluss]

The maintainer has to make the decisions, that's what we are for. Adding 59 crates as dev-dependencies is a no-go, optional or not.

[bluss]

Thanks anyway. I'll just close this. I have had hopes that criterion would replace "bencher" but I wouldn't use it in this state. A lot of the crate looks awesome, of course.