Add iterator versions of BTreeSet operations

[DevinR528]

The motivation for this is when using union, intersection, difference, and symmetric_difference the items have to be cloned, this avoids that and can return borrowed items. What I was shooting for was as if the signature to BTreeSet::intersection was

pub fn difference<'a>(&'a self, other: &'a BTreeSet<Q>) -> Difference<'a, Q>
where
    T: Borrow<Q>,
    Q: Ord,
{...}

The code is taken from the std lib and altered to work with iterators so it should be sound, still working out a few kinks but nothing major.

I did try implementing the above signature in the std lib but was unable (could be just me) so I tried this out and got it working! Let me know if this is helpful as part of itertools.

Thanks by the way for a great crate!

Huge problem

This relies on the iterator returning items in sorted order, is this an acceptable invariant to only document (is there any way to add a trait to signify that the iterator needs to be sorted?).