Implement timeouts, trim search horizon based on solution so far

src/main/scala/leon/purescala/Extractors.scala

src/main/scala/leon/synthesis/Complexity.scala

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

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

src/main/scala/leon/synthesis/Solution.scala

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

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

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