Made Phase a trait

... so that it can be combined with TreeTransform in a trait composition
in a future dientanglement of TreeTransforms and Phases.

Author: odersky

src/dotty/tools/dotc/core/Phases.scala

@@ -180,7 +180,7 @@ object Phases {
   final val erasureName = "erasure"
   final val flattenName = "flatten"
 
-  abstract class Phase extends DotClass {
+  trait Phase extends DotClass {
 
     def name: String