support for multisets extraction

project/build/funcheck.scala

src/funcheck/CodeExtraction.scala

src/funcheck/Extractors.scala

src/orderedsets/Main.scala

src/purescala/PrettyPrinter.scala

src/purescala/Trees.scala

src/purescala/TypeTrees.scala

src/purescala/Z3Solver.scala

testcases/MultisetOperations.scala