xlang does nothing unless needed

src/main/scala/leon/Main.scala

src/main/scala/leon/xlang/AntiAliasingPhase.scala

src/main/scala/leon/xlang/ArrayTransformation.scala

src/main/scala/leon/xlang/ImperativeCodeElimination.scala

src/main/scala/leon/xlang/XLangDesugaringPhase.scala