Make parameters to implicit value classes private

Rebase of #3046, first bumping starr to M6
(Left other versions as-is, as something seems wrong with partest.
Looking into that now.)

Old description:
So that they aren't offered as an autocomplete suggestion:

    implicit class Shouty(string: String) extends AnyVal {
     def SHOUT_! = string.toUpperCase + "!"
    }
    "". // autocompletion offers `.string` here.

The original incarnation of value classes didn't allow this sort of
encapsulation, so we either invented goofy names like
`__thingToAdd` or just picked `x` or `self`. But  has delivered us the
freedom to keep the accessor private.

Should we keep any of these accessors around in a deprecated form?

The implicit classes in Predef were added in 2.11.0-M2
(c26a8db067e4f), so they are okay.

I think we can make reason that these APIs were both accidental and unlikely
to be interpreted as public, so we can break them immediately.

    scala> Left(1).x
    res0: scala.util.Either[Int,Int] = Left(1)
    
    scala> import concurrent.duration._
    import concurrent.duration._
    
    scala> 1.n
    res1: Int = 1