Undo all changes to Tuples.

For the two operations that we really need for NamedTuple, we
define them as "internal" helpers inside NamedTuple.

We remove or amend tests that relied on the changes to Tuple. No
tests about NamedTuple's are affected.

library/src/scala/NamedTuple.scala

@@ -79,7 +79,7 @@ object NamedTuple:
     /** The tuple consisting of all elements of this tuple followed by all elements
      *  of tuple `that`. The names of the two tuples must be disjoint.
      */
-    inline def ++ [N2 <: Tuple, V2 <: Tuple](that: NamedTuple[N2, V2])(using Tuple.Disjoint[N, N2] =:= true)
+    inline def ++ [N2 <: Tuple, V2 <: Tuple](that: NamedTuple[N2, V2])(using FutureTupleOps.Disjoint[N, N2] =:= true)
       : NamedTuple[Tuple.Concat[N, N2], Tuple.Concat[V, V2]]
       = toTuple ++ that.toTuple
 
@@ -200,6 +200,30 @@ object NamedTuple:
   /** The empty named tuple */
   val Empty: Empty = EmptyTuple.asInstanceOf[Empty]
 
+  @experimental
+  object FutureTupleOps {
+
+    /** A type level Boolean indicating whether the type `Y` contains
+     *  none of the elements of `X`.
+     *  @pre  The elements of `X` and `Y` are assumed to be singleton types
+     */
+    type Disjoint[X <: Tuple, Y <: Tuple] <: Boolean = X match
+      case x *: xs => Contains[Y, x] match
+        case true => false
+        case false => Disjoint[xs, Y]
+      case EmptyTuple =>
+        true
+
+    /** A type level Boolean indicating whether the tuple `X` has an element
+     *  that matches `Y`.
+     *  @pre  The elements of `X` are assumed to be singleton types
+     */
+    type Contains[X <: Tuple, Y] <: Boolean = X match
+      case Y *: _ => true
+      case _ *: xs => Contains[xs, Y]
+      case EmptyTuple => false
+  }
+
 end NamedTuple
 
 /** Separate from NamedTuple object so that we can match on the opaque type NamedTuple. */
@@ -214,4 +238,3 @@ object NamedTupleDecomposition:
   /** The value types of a named tuple represented as a regular tuple. */
   type DropNames[NT <: AnyNamedTuple] <: Tuple = NT match
     case NamedTuple[_, x] => x
-