Add back unapply and copy, but marked as private

[alexarchambault]

It would be safe to still generate the unapply and copy methods, but with a private modifier. They could be used / helpful from the class body, but wouldn't belong to the class public API, so couldn't break binary compatibility.

[jvican]

This is a good idea. It would be great if we could tweak private to private[$package] so that library authors can still use unapply and copy internally inside their library if they want to.