Syntax for type tuple with one element

[sjrd]

@odersky

We had EmptyTuple = Unit originally, but this ran into issues. Maybe others can chime in what the issues were, my memory is a bit hazy here.

EmptyTuple = Unit was a no-go for Scala.js. In Scala.js, Unit must be JavaScript’s undefined value. We could not reconcile that with the fact that EmptyTuple needs to extend scala.Product.