New capture-vars syntax, "higher-kinds" style

Capture variables and members are marked
by the sticky `CaptureVar` attachment to
their TypeDef and the respective rhs.

Author: bracevac

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

@@ -642,7 +642,7 @@ object desugar {
       .withMods(mods & (GivenOrImplicit | Erased | hasDefault | Tracked) | Param)
   }
 
-  /** Desugar type def (not param): Under x.moduliity this can expand
+  /** Desugar type def (not param): Under x.modularity this can expand
    *  context bounds, which are expanded to evidence ValDefs. These will
    *  ultimately map to deferred givens.
    */

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

@@ -34,6 +34,9 @@ object Trees {
 
   val SyntheticUnit: Property.StickyKey[Unit] = Property.StickyKey()
 
+  /** Property key for marking capture-set variables and members */
+  val CaptureVar: Property.StickyKey[Unit] = Property.StickyKey()
+
   /** Trees take a parameter indicating what the type of their `tpe` field
    *  is. Two choices: `Type` or `Untyped`.
    *  Untyped trees have type `Tree[Untyped]`.