Phase out intersect(::AbstractInterval, ::AbstractInterval) -> AbstractInterval

[omus]

A single interval can be treated as a representation of a set. For the function intersect we added a method such that the intersection of any two intervals can return an instance of an interval. This is possible as the result of such an intersection will either be an interval or an empty interval.

However, the other set functions cannot work this way as there exist cases where multiple intervals would have to be returned:

• union(::Interval, ::Interval) -> Interval: returns multiple intervals for disjoint intervals
• setdiff(::Interval, ::Interval) -> Interval: return multiple intervals when the first argument is a superset of the second
• symdiff(::Interval, ::Interval) -> Interval: returns multiple intervals in all cases except for when the intervals are contiguous

I would suggest that we use a different name for the existing intersect method and have all of the set methods return a vector so they have consistent return arguments. This is more inline with the Base behaviour.

[omus]

I came to this realization while reviewing #179