Class fields don't need to me mutable

src/main/scala/leon/frontends/scalac/CodeExtraction.scala

src/main/scala/leon/purescala/DefOps.scala

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

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

src/main/scala/leon/purescala/RestoreMethods.scala