Remove explicit use of Tuple2 in backend

The idea is to have `type Tuple2[A, B] = (A, B) // Desugars into TupleCons[A, TupleCons[B, Unit]]` in Predef, so that users can refer to "a tuple of size 2" either with `(A, B)` or `Tuple2[A, B]`. It's still possible to refer to the actually underlying class with the explicit prefix, `scala.Tuple2`. Because this code is inside the scala package and mixes the two syntax in breaks with this PR...

Author: OlivierBlanvillain