sum-types: What should the private keyword do for constructors?

[RyanGlScott]

On the sum-types branch (#1602), it is possible to annotate a constructor as private. Currently, this does nothing, and private constructors are exported anyway. It's unclear if we actually want the ability to mark enum constructors as private however—what does it mean to only export some of an enum's constructors?

We should come to a conclusion about what design we want and either (1) implement the feature, or (2) remove it.

[RyanGlScott]

We opted to remove this for now in 8abf33f. We can always add this back later if someone requests it.