Macro rand_core::impl_try_rng_from_rng_core

macro_rules! impl_try_rng_from_rng_core {
    ($t:ty) => { ... };
}

Implement TryRngCore for a type implementing RngCore.

Ideally, rand_core would define blanket impls for this, but they conflict with blanket impls for &mut R and Box<R>, so until specialziation is stabilized, implementer crates have to implement TryRngCore directly.