Introduce old notation for vars in postcondition

library/lang/package.scala

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

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

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

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

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

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

...es/regression/verification/xlang/valid/ArrayNested2.scala

...rces/regression/verification/xlang/valid/NestedOld1.scala

...rces/regression/verification/xlang/valid/NestedOld2.scala