Min/Max value?

[rljacobson]

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

  #[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.