Remove activation of rand_core/std

[tarcieri]

This feature provides two things:

1. Transitive activation of the rand_core/getrandom feature
2. Impl of std::error::Error for rand_core::Error

rand_core::Error is not part of curve25519-dalek's public API and is irrelevant.

The rand_core/getrandom feature provides OsRng which is used in tests only, so we can activate it in dev-dependencies.

[rozbb]

rand_core/std does one thing: impl<R: RngCore> RngCore for Box<R>. Theoretically a user might care about this trait.

But if we remove rand_core/std anyways, then we can remove std entirely, since subtle/std does nothing.

[rozbb]

Actually that's wrong. The README says that, but in reality you only need alloc. So I guess we can remove std altogether

[tarcieri]

Oh hey, you're right. Let me open a PR based on this one that removes the std feature entirely

Edit: opened #459