rust-dirstate: call rust dirstatemap from Python Since Rust-backed Python classes cannot be used as baseclasses (for rust-cpython anyway), we use composition rather than inheritance. This also allows us to keep the IO operations in the Python side, removing (for now) the need to rewrite VFS in Rust, which would be a heavy undertaking. Differential Revision: https://phab.mercurial-scm.org/D6634