Common Tree class, rework & improve pretty-printers

src/main/scala/leon/purescala/Common.scala

src/main/scala/leon/purescala/Definitions.scala

src/main/scala/leon/purescala/PrettyPrinter.scala

src/main/scala/leon/purescala/ScalaPrinter.scala

src/main/scala/leon/purescala/Trees.scala

src/main/scala/leon/purescala/TypeTrees.scala

src/main/scala/leon/synthesis/FileInterface.scala

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

src/main/scala/leon/xlang/Trees.scala