rust-pyo3: dagop submodule implementation...
rust-pyo3: dagop submodule implementation
This is the first demonstration that the passing of objects through the
Python interpreter from `rustext` to `pyo3_rustext` actually works.
In the Python tests, we conflate the presence of the `pyo3_rustext` package
with that of `rustext`, as we do not plan to support building one and not
the other (we hope to convert fully to PyO3 soon).
The skipping is actually done by the base test class.
The implementation of `rank` is as trivial as it was in `hg-cpython`,
yet it required to be able to convert exceptions from `vcsgraph`.
