Proposal: use criterion crate instead of bencher crate in benchmark

[wathenjiang]

The current benchmark crate is bencher, but the criterion is more modern and being maintained, which can be found here.

I would like to do more benchmarks, but I found the API of bencher is kind of poor, such as the lack of iter_custom in criterion.

If this proposal can be passed, I would like to help compelete it.

[Darksonn]

I don't mind changing the benchmarks to criterion. Go ahead and file a PR if you want to.