move definition of Domain type to DomainSetsCore.jl?

[daanhb]

I had made a minimal implementation of DomainSetsCore.jl, a lightweight package that defines the Domain type and a domain interface. I've updated the package this week: see https://github.com/JuliaApproximation/DomainSetsCore.jl.

As things stand, I've checked that this package is compatible with IntervalSets (see master...daanhb:IntervalSets.jl:core) and with DomainSets versions 0.6 and 0.7. Moving the definition of Domain{T} seems a non-breaking change, though it would have to be coordinated so that IntervalSets and DomainSets can both be updated.

In principle, with the changes being made, IntervalSets could later go on without actually inheriting from Domain{T}. That would also require some more changes in DomainSets to ensure that such a change is seamless, so not just yet please.

Thoughts? First step would be to register DomainSetsCore.jl.

(pinging @dlfivefifty)