Add set operations

[apasel422]

The traits can be extended with additional methods that yield the difference and symmetric difference between two ordered iterators. The union is already handled by outer_join, and the intersection is already handled by inner_join.

[apasel422]

I think we should rename the join methods to their appropriate set operation name.

[Gankra]

It's worth noting that these are established terms in relational algebra. I have no strong feelings on the names, otherwise.

[apasel422]

Is there a difference other than the names? I've been thinking that it's silly for every (ordered) set implementation to define its own {Difference, Intersection, SymmetricDifference, Union} iterators. It would make sense for this crate to provide a generic implementation as extension methods. The same goes for ordered map iterators.

Note that this is different from optimized set operations that return a new set (e.g. BitSet).

[Gankra]

That... is an interesting idea. It's not clear to me that the optimal impl is necessarily blanket-able in general, though.

[apasel422]

Another reason I favor the generic iterator adaptors here is that it avoids the theoretical combinatoric problem with iteration modes. Right now, the the sets in std and contain-rs only provide by-ref iterators, but there's no reason they couldn't have by-value or by-mut-ref ones, too.

[apasel422]

Not to mention the bounded iterators.