Promote LetDef to purescala

src/main/scala/leon/Main.scala

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

src/main/scala/leon/purescala/FunctionClosure.scala

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/purescala/Trees.scala

src/main/scala/leon/synthesis/CostModel.scala

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

src/main/scala/leon/xlang/TreeOps.scala

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

src/main/scala/leon/xlang/XlangAnalysisPhase.scala