Existential Capabilities (scala#20566)

The design has changed quite a bit relative to
scala#20470. It is written up as a doc
comment on object cc.Existentials.

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -521,12 +521,17 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   def captureRoot(using Context): Select =
     Select(scalaDot(nme.caps), nme.CAPTURE_ROOT)
 
-  def captureRootIn(using Context): Select =
-    Select(scalaDot(nme.caps), nme.capIn)
-
   def makeRetaining(parent: Tree, refs: List[Tree], annotName: TypeName)(using Context): Annotated =
     Annotated(parent, New(scalaAnnotationDot(annotName), List(refs)))
 
+  def makeCapsOf(id: Ident)(using Context): Tree =
+    TypeApply(Select(scalaDot(nme.caps), nme.capsOf), id :: Nil)
+
+  def makeCapsBound()(using Context): Tree =
+    makeRetaining(
+      Select(scalaDot(nme.caps), tpnme.CapSet),
+      Nil, tpnme.retainsCap)
+
   def makeConstructor(tparams: List[TypeDef], vparamss: List[List[ValDef]], rhs: Tree = EmptyTree)(using Context): DefDef =
     DefDef(nme.CONSTRUCTOR, joinParams(tparams, vparamss), TypeTree(), rhs)