Always pass monitors, improve them

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

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

src/main/scala/leon/codegen/CompiledExpression.scala

src/main/scala/leon/codegen/runtime/Monitor.scala

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

src/main/scala/leon/evaluators/CodeGenEvaluator.scala

src/main/scala/leon/evaluators/DualEvaluator.scala