Add SeqSet

[NthPortal]

Add SeqSet, mirroring SeqMap.

[NthPortal]

Note: without adding new implementations for the new traits, this depends on #21

[NthPortal]

thoughts on this change? (👍 in favour, 👎 against)

[julienrf]

So, we would have LinkedHashSet as a concrete subtype of SeqSet?

[NthPortal]

yes, although it will take some fiddling to make that actually work in scala-library-next (if it's not final, we might be able to subtype it with a mixin?)

[SethTisue]

We'd want VectorSet as well: #168