We'll probably end up adding more automatically-solved classes after 1.0, so I think it makes sense to move them into submodules of Prim before 1.0, and not import those modules automatically.

For example, Union and RowCons could move into Prim.Rows (and we'd rename RowCons to just Cons while we're at it, probably).

I'll put this under 1.0 for now.