rust-index: stop using C index...
rust-index: stop using C index
We still keep its wrapper implementation in `hg-cpython::cindex`,
because we might want to recreate ancestors handling objects using
it for the case of REVLOGV2.
Also, we still instantiate it (from Python code) and store it as
attribute, for the likes of `get_cindex` and the caller that
relies on it, but that is soon to be removed, too.
