Make parameters to implicit value classes private

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 SI-7859 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