XLang is now a desugaring phase

src/main/scala/leon/Main.scala

src/main/scala/leon/SharedOptions.scala

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

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

src/main/scala/leon/verification/VerificationReport.scala

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

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

src/main/scala/leon/xlang/FixReportLabels.scala

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

src/main/scala/leon/xlang/XLangAnalysisPhase.scala

...scala/leon/test/verification/XLangVerificationSuite.scala