This changeset introduces the hg-cpython crate,
that compiles as a shared library holding a whole
Python package (mercurial.rustext), with only the empty
'ancestor' submodule for now.
Such bindings will be easier and safer to develop and maintain
that those of hg-direct-ffi.
They don't involve C code, only unsafe Rust that's mostly isolated
within the cpython crate.
The long-term goal would be to import the provided modules, such
as rustext.ancestor with mercurial.policy.importmod, same as
we already do with cext modules.