rust-dirstate: add rust implementation of `parse_dirstate` and `pack_dirstate` Working towards the goal of having a complete Rust implementation of `hg status`, these two utils are a first step of many to be taken to improve performance and code maintainability. Two dependencies have been added: `memchr` and `byteorder`. Both of them have been written by reputable community members and are very mature crates. The Rust code will often need to use their byte-oriented functions. A few unit tests have been added and may help future development and debugging. In a future patch that uses `parse_dirstate` to stat the working tree in parallel - which neither the Python nor the C implementations do - actual performance improvements will be seen for larger repositories. Differential Revision: https://phab.mercurial-scm.org/D6348