cext: increment the `parsers` module version We introduced a new `slicechunktodensity` method to the index.

sparse-revlog: introduce native (C) implementation of slicechunktodensity This is a C implementation of `_slicechunktodensity` in the `mercurial/revlogutils/deltas.py` file. The algorithm involves a lot of integer manipulation and low-level access to index data. Having a C implementation of it raises a large performance improvement. See later changeset in this series for details.