Type hints & PEP 561 packaging

Add type hints to all (at least public API) code make the distribution PEP 561 compliant. This allows `mypy` (and other tools?) to find the type hints and use them in linting.

In practice, add an empty `awscrt/py.typed` file, and include it in the package:

```py
setup(
package_data={'awscrt': ['py.typed']},
zip_safe=False, # not needed with wheels, AFAIK
)
```

PEP 561: https://www.python.org/dev/peps/pep-0561/