HashSet operations against another HashSet should allow for differing hash algorithms

[shepmaster]

Operations such as intersection take a second HashMap. It's required that the second HashMap implement the same hashing algorithm as the first one:

impl<T, S> HashSet<T, S>
    where T: Eq + Hash,
          S: HashState
{
    fn intersection<'a>(&'a self, other: &'a HashSet<T, S>) -> Intersection<'a, T, S>;
}

This means that you cannot intersect a HashMap<u8, SipHash> against a HashMap<u8, FnvHash>. This seems overly restrictive.

/cc @bluss

[bluss]

With FnvHash it doesn't need to recompute hashes either. But we should not stall on that, this change is more likely to introduce type inference regressions the longer custom hasher support is stable.

[steveklabnik]

Triage: not aware of any changes here