rust-dirstate: add "dirs" Rust implementation Following the work done in d1786c1d34fa and working towards the goal of a complete Rust implementation of the dirstate, this rewrites the `dirs` class. There is already a C implementation, which relies heavily on CPython hacks and protocol violations for performance, so I don't expect this to perform as well for now, as this is very straight-forward code. The immediate benefits are new high-level documentation and some unit tests. Differential Revision: https://phab.mercurial-scm.org/D6393