non-parametric interval for type stability

I have found myself dealing with unions of disjoint intervals sufficiently often that I feel it's about time I wrote a package for it. I think it's good for that to be a separate package from this, partially because unions of `Interval` are not closed, so my package will have its own union function that always returns the union type.

It quickly became clear to me that I'm going to need a union type which does not specify its openness via type parameters for anything in my package to be type stable (in particular even the containers could not be type stable for `Interval`). I've been calling this type `GenericInterval{T} <: AbstractInterval{T}`. Would there be any interest in me adding that to this package instead of putting it in my own?

As an aside, I'm self conscious of the whole `DomainSets` thing, and not super comfortable with what I'm doing, particularly as my implementation so far only supports unions of finitely many intervals, but I figure if I don't restrain myself I'm going to wind up spending 5 years writing a package that describes arbitrary sets on arbitrary manifolds of any dimension.