Add deprecated RngCore/TryRngCore forwarding traits

[tarcieri]

Since RngCore has been renamed to Rng, and TryRngCore to TryRng, this adds some deprecated stub traits with the old names that have a note about the new names.

They have supertrait bounds on the new names, along with blanket impls for the new names:

• RngCore: Rng
• TryRngCore: TryRng

These can be released in v0.10.x and removed before v1.x.

Though they don't have any methods, due to an odd quirk of how Rust resolves methods in a generic context (effectively uniting a trait and all of its supertraits into a single logical method table), the methods of the new traits are callable from a generic context even without the new traits in the bounds or even in scope whatsoever:

use rand_core::RngCore;

pub fn foo<R: RngCore>(mut rng: R) -> [u8; 8] {
    let mut ret = [0u8; 8];
    rng.fill_bytes(&mut ret);
    ret
}

This will actually compile with the following warnings:

warning: use of deprecated trait `rand_core::RngCore`: use `Rng` instead
 --> src/main.rs:1:16
  |
1 | use rand_core::RngCore;
  |                ^^^^^^^
  |
  = note: `#[warn(deprecated)]` on by default

warning: use of deprecated trait `rand_core::RngCore`: use `Rng` instead
 --> src/main.rs:3:15
  |
3 | pub fn foo<R: RngCore>(mut rng: R) -> [u8; 8] {
  |               ^^^^^^^

I think this will help people upgrade because it will allow a significant amount of code to continue to compile while also informing people of the new names and what code needs to be changed.

[tarcieri]

The same trick doesn't apply to associated types though, so I forwarded this one.

[dhardy]

This doesn't help when it comes to implementing RNGs, but should help upgrade use-sites at least.

[dhardy]

It would also be useful to have a deprecation error for the methods SeedableRng::{try_,}from_os_rng; unfortunately we can't implement them (except by panicking).