Tentative implementation of multi-steps rules

src/main/scala/leon/synthesis/DerivationTree.scala

src/main/scala/leon/synthesis/Heuristics.scala

src/main/scala/leon/synthesis/Rules.scala

src/main/scala/leon/synthesis/SynthesisPhase.scala

src/main/scala/leon/synthesis/Synthesizer.scala

src/main/scala/leon/synthesis/Task.scala