Introduce type-private access level

[ole (Ole Begemann)]

@lmcd

Also, it's always seemed a bit bizarre for me how access control revolves around files in Swift, although I'm sure there are well thought out reasons for it.

I like @beccadax's take on this in a previous thread on "typeprivate":

So it’s precisely the fact that files are an arbitrary boundary that makes them good for access control. An arbitrary boundary is one you can draw around whatever you want—and that’s perfect for access control.