Refactor plugin into its phase

src/main/scala/leon/LeonContext.scala

src/main/scala/leon/LeonPhase.scala

src/main/scala/leon/Main.scala

src/main/scala/leon/plugin/AnalysisComponent.scala

src/main/scala/leon/plugin/ExtractorPhase.scala

src/main/scala/leon/plugin/LeonPlugin.scala

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