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

1 files changed, 1 insertions, 5 deletions

diff --git a/test/files/presentation/callcc-interpreter.check b/test/files/presentation/callcc-interpreter.check
index 59c841a255..d41b982614 100644
--- a/test/files/presentation/callcc-interpreter.check
+++ b/test/files/presentation/callcc-interpreter.check
@@ -3,7 +3,7 @@ reload: CallccInterpreter.scala
 askTypeCompletion at CallccInterpreter.scala(51,38)
 ================================================================================
 [response] askCompletionAt (51,38)
-retrieved 63 members
+retrieved 59 members
 abstract trait Term extends AnyRef
 abstract trait Value extends AnyRef
 case class Add extends callccInterpreter.Term with Product with Serializable
@@ -52,10 +52,6 @@ final def synchronized[T0](x$1: T0): T0
 final def wait(): Unit
 final def wait(x$1: Long): Unit
 final def wait(x$1: Long,x$2: Int): Unit
-private[this] val __leftOfArrow: callccInterpreter.type
-private[this] val __resultOfEnsuring: callccInterpreter.type
-private[this] val __stringToFormat: callccInterpreter.type
-private[this] val __thingToAdd: callccInterpreter.type
 private[this] val term0: callccInterpreter.App
 private[this] val term1: callccInterpreter.App
 private[this] val term2: callccInterpreter.Add