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, 5 insertions, 25 deletions

diff --git a/test/files/presentation/visibility.check b/test/files/presentation/visibility.check
index 4ba7dbaad9..078e0a2342 100644
--- a/test/files/presentation/visibility.check
+++ b/test/files/presentation/visibility.check
@@ -3,7 +3,7 @@ reload: Completions.scala
 askTypeCompletion at Completions.scala(14,12)
 ================================================================================
 [response] askCompletionAt (14,12)
-retrieved 41 members
+retrieved 37 members
 [inaccessible] private[this] def secretPrivateThis(): Unit
 def +(other: String): String
 def ->[B](y: B): (accessibility.Foo, B)
@@ -34,10 +34,6 @@ final def wait(): Unit
 final def wait(x$1: Long): Unit
 final def wait(x$1: Long,x$2: Int): Unit
 private def secretPrivate(): Unit
-private[this] val __leftOfArrow: accessibility.Foo
-private[this] val __resultOfEnsuring: accessibility.Foo
-private[this] val __stringToFormat: accessibility.Foo
-private[this] val __thingToAdd: accessibility.Foo
 protected def secretProtected(): Unit
 protected[package accessibility] def secretProtectedInPackage(): Unit
 protected[package lang] def clone(): Object
@@ -47,7 +43,7 @@ protected[package lang] def finalize(): Unit
 askTypeCompletion at Completions.scala(16,11)
 ================================================================================
 [response] askCompletionAt (16,11)
-retrieved 41 members
+retrieved 37 members
 def +(other: String): String
 def ->[B](y: B): (accessibility.Foo, B)
 def ensuring(cond: Boolean): accessibility.Foo
@@ -78,10 +74,6 @@ final def wait(x$1: Long): Unit
 final def wait(x$1: Long,x$2: Int): Unit
 private def secretPrivate(): Unit
 private[this] def secretPrivateThis(): Unit
-private[this] val __leftOfArrow: accessibility.Foo
-private[this] val __resultOfEnsuring: accessibility.Foo
-private[this] val __stringToFormat: accessibility.Foo
-private[this] val __thingToAdd: accessibility.Foo
 protected def secretProtected(): Unit
 protected[package accessibility] def secretProtectedInPackage(): Unit
 protected[package lang] def clone(): Object
@@ -91,7 +83,7 @@ protected[package lang] def finalize(): Unit
 askTypeCompletion at Completions.scala(22,11)
 ================================================================================
 [response] askCompletionAt (22,11)
-retrieved 41 members
+retrieved 37 members
 [inaccessible] private def secretPrivate(): Unit
 def +(other: String): String
 def ->[B](y: B): (accessibility.AccessibilityChecks, B)
@@ -122,10 +114,6 @@ 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: accessibility.AccessibilityChecks
-private[this] val __resultOfEnsuring: accessibility.AccessibilityChecks
-private[this] val __stringToFormat: accessibility.AccessibilityChecks
-private[this] val __thingToAdd: accessibility.AccessibilityChecks
 protected def secretProtected(): Unit
 protected[package accessibility] def secretProtectedInPackage(): Unit
 protected[package lang] def clone(): Object
@@ -135,7 +123,7 @@ protected[package lang] def finalize(): Unit
 askTypeCompletion at Completions.scala(28,10)
 ================================================================================
 [response] askCompletionAt (28,10)
-retrieved 41 members
+retrieved 37 members
 [inaccessible] private def secretPrivate(): Unit
 [inaccessible] private[this] def secretPrivateThis(): Unit
 [inaccessible] protected def secretProtected(): Unit
@@ -169,17 +157,13 @@ 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: accessibility.Foo
-private[this] val __resultOfEnsuring: accessibility.Foo
-private[this] val __stringToFormat: accessibility.Foo
-private[this] val __thingToAdd: accessibility.Foo
 protected[package accessibility] def secretProtectedInPackage(): Unit
 ================================================================================
 
 askTypeCompletion at Completions.scala(37,8)
 ================================================================================
 [response] askCompletionAt (37,8)
-retrieved 41 members
+retrieved 37 members
 [inaccessible] private def secretPrivate(): Unit
 [inaccessible] private[this] def secretPrivateThis(): Unit
 [inaccessible] protected def secretProtected(): Unit
@@ -214,8 +198,4 @@ 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: accessibility.Foo
-private[this] val __resultOfEnsuring: accessibility.Foo
-private[this] val __stringToFormat: accessibility.Foo
-private[this] val __thingToAdd: accessibility.Foo
 ================================================================================