Update rand 0.7 & criterion 0.4 and disable OS random number generator

[repi]

Disabled default features on rand to avoid pulling in things that are not needed, such as the OS random number generator in getrandom.

Want to be able to build for wasm32-unknown-unknown without having this pulled in because if called it will just panic (as there is no OS).

Unfortunately I think in practice due to criterion using rand with all features it will enable all features here also even though criterion is just a dev-dependency. But that is a separate cargo issue for now.

And good to use latest rand 0.7 anyway with minimal features.

And I do agree with #6 that ideally both rand and approx should be disabled by default.

[bitshifter]

Thanks again. I'd like to make the rand dependency optional at some point. I just need to remove the dependency on it in tests and bench.