rand: deprecate `rng`.
rand: deprecate `rng`.

This should be called far less than it is because it does expensive OS
interactions and seeding of the internal RNG, `task_rng` amortises this
cost. The main problem is the name is so short and suggestive.

The direct equivalent is `StdRng::new`, which does precisely the same
thing.

The deprecation will make migrating away from the function easier.

nixpulvis/uuid,tomprogrammer/uuid,rust-lang-nursery/uuid,mathstuf/rust-uuid,rust-lang/uuid