Library changes

[richsalz]

This is a companion to #55

It would be nice to have an os::get_entropy call that did the right (FIPS-compatible) thing. See https://github.com/openssl/openssl/tree/master/providers/implementations/rands/seeding for examples.

[newpavlov]

I think the getrandom crate already covers it.

[richsalz]

@newpavlov you are right, it does! I wonder if that should be added to the crypto list?