Capability-based random number generators

This corresponds to rand.

Capability-based APIs represent access to external resources as values which can be passed around between different parts of a program.

Two notable features are the OsRng and CapRng types, which wrap up access to the operating system entropy source in capability values.

This crate uses the existing rand::SeedableRng trait rather than having its own version, however while rand::SeedableRng is mostly just a pure interface, it provides a from_entropy function which directly reads from the operating system entropy source. To preserve the capability-based interface, avoid using rand::SeedableRng's from_entropy function on any of the types that implement that trait; use std_rng_from_entropy instead.

The cap-rand crate provides a capability-based interface to random number generators via the rand crate.