bench: typecheck the gallery before extraction/execution
I know that extraction and execution are much more fun than the boring "can we even parse WhyML at all?" question, but you see, if our parsing/typechecking is broken, then the fun stuff is broken, too. And I really prefer to get the error messages about the typechecking being broken from the typechecking part of the bench, and not from the test-api or extraction part. Merci pour votre compréhension.