support for functions with impure bodies

src/funcheck/CodeExtraction.scala

src/funcheck/Extractors.scala

src/funcheck/FunCheckPlugin.scala

src/multisets/Main.scala

src/purescala/Analysis.scala

src/purescala/Definitions.scala

src/purescala/PrettyPrinter.scala

src/purescala/Reporter.scala

testcases/IntOperations.scala