rust-dirstate: add rust-cpython bindings to the new parse/pack functions This allows for Python code to call `parse/pack_dirstate` transparently. These bindings are heavy given the relatively simple task, as they are bound to implementation details of both the C and Python code. They will be slimmed down in future patches and eventually completely removed once more of the dirstate code has been refactored/rewritten in Rust. Both functions emulate the mutate-on-loop style of the Python and C implementations by looping over changed items in the compatibility layer, instead of at the core functions. Differential Revision: https://phab.mercurial-scm.org/D6349