The information that was contained in `schema.yml` is now in
`swift/schema.py`, which allows a more integrated IDE experience
for writing and navigating it.
Another minor change is that `schema.Class` now has a `str` `group`
field instead of a `pathlib.Path` `dir` one.
Tests can be run with
```
bazel test //swift/codegen:tests
```
Coverage can be checked installing `pytest-cov` and running
```
pytest --cov=swift/codegen swift/codegen/test
```
