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, 2 insertions, 2 deletions

diff --git a/test/files/neg/logImplicits.check b/test/files/neg/logImplicits.check
index 0522bd8354..2265614962 100644
--- a/test/files/neg/logImplicits.check
+++ b/test/files/neg/logImplicits.check
@@ -7,10 +7,10 @@ logImplicits.scala:7: applied implicit conversion from String("abc") to ?{def ma
 logImplicits.scala:15: inferred view from String("abc") to Int = C.this.convert:(p: String("abc"))Int
   math.max(122, x: Int)
                 ^
-logImplicits.scala:19: applied implicit conversion from Int(1) to ?{def ->: ?} = implicit def ArrowAssoc[A](__leftOfArrow: A): ArrowAssoc[A]
+logImplicits.scala:19: applied implicit conversion from Int(1) to ?{def ->: ?} = implicit def ArrowAssoc[A](self: A): ArrowAssoc[A]
   def f = (1 -> 2) + "c"
            ^
-logImplicits.scala:19: applied implicit conversion from (Int, Int) to ?{def +: ?} = implicit def StringAdd[A](__thingToAdd: A): StringAdd[A]
+logImplicits.scala:19: applied implicit conversion from (Int, Int) to ?{def +: ?} = implicit def StringAdd[A](self: A): StringAdd[A]
   def f = (1 -> 2) + "c"
              ^
 logImplicits.scala:22: error: class Un needs to be abstract, since method unimplemented is not defined