Fixed evaluators and data-gen for bags

src/main/java/leon/codegen/runtime/Bag.java

src/main/java/leon/codegen/runtime/BigInt.java

src/main/scala/leon/codegen/CodeGeneration.scala

src/main/scala/leon/codegen/CompilationUnit.scala

src/main/scala/leon/datagen/VanuatooDataGen.scala

src/main/scala/leon/evaluators/RecursiveEvaluator.scala

src/main/scala/leon/evaluators/StreamEvaluator.scala

src/main/scala/leon/solvers/unrolling/UnrollingSolver.scala

...st/scala/leon/integration/evaluators/EvaluatorSuite.scala