Implement ExactSizeIterator for accessor::util::Iter

[aloucks]

This trait was implemented for Iter prior to 0.14. It appears that
the missing trait implementation was an oversight when sparse
accessors were implemented in #246.

[alteous]

Thanks for the PR! It's not an oversight per se, I didn't think it made sense at the time. After re-reading the ExactSizeIterator docs I don't think this implementation is appropriate.

I think we would need to do something like this instead:

impl<'a, T: Item> ExactSizeIterator for SparseIter<'a, T> {}
impl<'a, T: Item> Iterator for SparseIter<'a, T> {
    ...
    fn size_hint(&self) -> usize {
        let hint = self.values.len() - self.counter;
        (hint, Some(hint))
    }
}

[aloucks]

@alteous I've updated this based on your feedback and also added the other methods to Iter that were implemented for ItemIter.

[alteous]

LGTM, thanks. 👍