Rename XLang Trees

src/main/scala/leon/evaluators/RecursiveEvaluator.scala

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

src/main/scala/leon/solvers/z3/AbstractZ3Solver.scala

src/main/scala/leon/verification/InjectAsserts.scala

src/main/scala/leon/xlang/ArrayTransformation.scala

src/main/scala/leon/xlang/EpsilonElimination.scala

src/main/scala/leon/xlang/ExprOps.scala

src/main/scala/leon/xlang/Expressions.scala

src/main/scala/leon/xlang/ImperativeCodeElimination.scala

src/main/scala/leon/xlang/NoXLangFeaturesChecking.scala