Skip should implement ExactSizeIterator

[awelkie]

This should be in iter.rs:

impl<I> ExactSizeIterator for Skip<I> where I: ExactSizeIterator{ }

This would require either implementing DoubleEndedIterator for Skip<I> (where I: DoubleEndedIterator), or decoupling ExactSizeIterator and DoubleEndedIterator (like in #19395)