Unify XOperators. Eliminate dead trees

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

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

src/main/scala/leon/purescala/Constructors.scala

src/main/scala/leon/purescala/ExprOps.scala

src/main/scala/leon/purescala/Expressions.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/ScopeSimplifier.scala

src/main/scala/leon/purescala/TransformerWithPC.scala

src/main/scala/leon/purescala/TypeOps.scala

src/main/scala/leon/purescala/Types.scala

...main/scala/leon/solvers/templates/TemplateGenerator.scala

src/main/scala/leon/solvers/templates/Templates.scala

src/main/scala/leon/synthesis/Witnesses.scala

src/main/scala/leon/synthesis/utils/ExpressionGrammar.scala

src/main/scala/leon/utils/UnitElimination.scala

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

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

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