Min/Max value?

It would be nice to have `min` and `max` methods. I believe `min` can be implemented as:

```rust
  #[inline(always)]
  pub fn min_value(&self) -> Option<usize> {
    self.0.iter().next()
  }
```

It's not clear how to efficiently implement `max`.

If the iterator implemented `DoubleEndedIterator`, then `max` could be implemented the same way.