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, 3 insertions, 15 deletions

diff --git a/test/files/presentation/ide-bug-1000475.check b/test/files/presentation/ide-bug-1000475.check
index e4b8508846..cb7de6d34a 100644
--- a/test/files/presentation/ide-bug-1000475.check
+++ b/test/files/presentation/ide-bug-1000475.check
@@ -3,7 +3,7 @@ reload: Foo.scala
 askTypeCompletion at Foo.scala(3,7)
 ================================================================================
 [response] askCompletionAt (3,7)
-retrieved 35 members
+retrieved 31 members
 [inaccessible] protected[package lang] def clone(): Object
 [inaccessible] protected[package lang] def finalize(): Unit
 def +(other: String): String
@@ -32,16 +32,12 @@ 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: Object
-private[this] val __resultOfEnsuring: Object
-private[this] val __stringToFormat: Object
-private[this] val __thingToAdd: Object
 ================================================================================
 
 askTypeCompletion at Foo.scala(6,10)
 ================================================================================
 [response] askCompletionAt (6,10)
-retrieved 35 members
+retrieved 31 members
 [inaccessible] protected[package lang] def clone(): Object
 [inaccessible] protected[package lang] def finalize(): Unit
 def +(other: String): String
@@ -70,16 +66,12 @@ 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: Object
-private[this] val __resultOfEnsuring: Object
-private[this] val __stringToFormat: Object
-private[this] val __thingToAdd: Object
 ================================================================================
 
 askTypeCompletion at Foo.scala(7,7)
 ================================================================================
 [response] askCompletionAt (7,7)
-retrieved 35 members
+retrieved 31 members
 [inaccessible] protected[package lang] def clone(): Object
 [inaccessible] protected[package lang] def finalize(): Unit
 def +(other: String): String
@@ -108,8 +100,4 @@ 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: Object
-private[this] val __resultOfEnsuring: Object
-private[this] val __stringToFormat: Object
-private[this] val __thingToAdd: Object
 ================================================================================