Enable all stable features in the playground

Fix #1082 (seed_from_u64 with non multiple of 4)

This commit fixes a panic when generating a single sample in an
inclusive range that spans the entire integer range, eg for u8:
```rust
rng.gen_range(0..=u8::MAX)
// panicked at 'attempt to add with overflow', src/distributions/uniform.rs:529:42
```
[Playground example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&code=use%20rand%3A%3ARng%3B%0A%0Afn%20main()%20%7B%0A%20%20%20%20rand%3A%3Athread_rng().gen_range(0u8..%3D255u8)%3B%0A%7D).

The cause is a discrepancy between the "single sample" and the "many
samples" codepaths:
```rust
// Ok
UniformSampler::new_inclusive(u8::MIN, u8::MAX).sample(&mut rng);
// Panic
UniformSampler::sample_single_inclusive(u8::MIN, u8::MAX, &mut rng);
```
In `sample`, a `range` of 0 is interpreted to mean "sample from the
whole range".
In `sample_range_inclusive`, no check is performed, which leads to
overflow when computing the `ints_to_reject`.

**Testing**
- Added a test case.
- Old code panics, new code passes.

…_full_inclusive_range

distributions/uniform: fix panic in gen_range(0..=MAX)