Rework random number generation

Switch core to taking a trait instead of a function This allows storing RNG data with the Context Switch cli from rand::rngs::ThreadRng (secure) to nanorand::WyRand (fast) --- In pyfend, I'm doing the same awkward static thing https://github.com/raylu/pyfend/blob/a751c056791acd030cf18499d470876de4d6a129/src/lib.rs#L17 as rand is https://github.com/rust-random/rand/blob/0c955c5b7a079bc2fe67fe946a8deb46c4bc58d8/src/rngs/thread.rs#L158