Fall back to previous implementation scheme for MegaPhase

The new implementation is essentially a slightly cleaned-up version of the
old one from TreeTransformers, which seemed to performed better. The main difference (besides
renaming things) is that prepare operations still return contexts instead of TreeTransforms.

Author: odersky

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -97,9 +97,9 @@ class Compiler {
       List(new LinkScala2Impls,     // Redirect calls to trait methods defined by Scala 2.x, so that they now go to their implementations
            new LambdaLift,          // Lifts out nested functions to class scope, storing free variables in environments
                                        // Note: in this mini-phase block scopes are incorrect. No phases that rely on scopes should be here
-           new ElimStaticThis),      // Replace `this` references to static objects by global identifiers
+           new ElimStaticThis),     // Replace `this` references to static objects by global identifiers
       List(new Flatten,             // Lift all inner classes to package scope
-           new RestoreScopes,      // Repair scopes rendered invalid by moving definitions in prior phases of the group
+           new RestoreScopes,       // Repair scopes rendered invalid by moving definitions in prior phases of the group
            new RenameLifted,        // Renames lifted classes to local numbering scheme
            new TransformWildcards,  // Replace wildcards with default values
            new MoveStatics,         // Move static methods to companion classes

compiler/src/dotty/tools/dotc/transform/LambdaLift.scala

@@ -491,6 +491,7 @@ class LambdaLift extends MiniPhase with IdentityDenotTransformer { thisPhase =>
   val phaseName: String = "lambdaLift"
 
   override def relaxedTypingInGroup = true
+    // Because it adds free vars as additional proxy parameters
 
   override def runsAfterGroupsOf: Set[Class[_ <: Phase]] = Set(classOf[Constructors], classOf[HoistSuperArgs])
     // Constructors has to happen before LambdaLift because the lambda lift logic