Update RNG performance stats

[dhardy]

The rngs repository benchmarks have been updated to use Criterion and report cycles-per-byte (rust-random/rngs#34); we likely use that, either duplicating the framework to the rand repository or by adding dev-dependencies in rngs to all the RNGs we want to benchmark.

We should wait until the next rand release, at least to get all RNGs on the same version of rand_core.