Fix printing bugs, add synthesis->print tests

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

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

src/main/scala/leon/purescala/TreeOps.scala

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

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

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

src/test/scala/leon/test/synthesis/StablePrintingSuite.scala