Fix many issues related to web-interface

project/Build.scala

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

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

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

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

src/main/scala/leon/synthesis/rules/Cegis.scala

src/main/scala/leon/utils/Simplifiers.scala

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