Improve linting
- Add PEP 561 py.typed marker file indicating that chutney has inline type annotations, so that mypy can check code that uses chutney, including our examples.
- Fix type issues in our example: use dataclasses.replace to get type checking, and fix instances of int where bool is expected.
- In CI, run mypy and lints over the whole repo, including examples, instead of just the chutney library.