rust-dirstate: use EntryState enum instead of literals This improves code readability quite a bit, while also adding a layer of safety because we're checking the state byte against the enum. Differential Revision: https://phab.mercurial-scm.org/D6629

rust-parsers: switch to parse/pack_dirstate to mutate-on-loop Both `parse_dirstate` and `pack_dirstate` can operate directly on the data they're passed, which prevents the creation of intermediate data structures, simplifies the function signatures and reduces boilerplate. They are exposed directly to the Python for now, but a later patch will make use of them inside `hg-core`. Differential Revision: https://phab.mercurial-scm.org/D6628

rust-parsers: move parser bindings to their own file and Python module This tidies up the Rust side while simplifying the Python side. Differential Revision: https://phab.mercurial-scm.org/D6627