rust-dirstate: rust-cpython bridge for dirstatemap This change also showcases the limitations of the `py_shared_ref!` macro. See the previous commit 'rust-dirstate: rust implementation of dirstatemap` for an explanation for the TODOs in the code. Differential Revision: https://phab.mercurial-scm.org/D6633

rust-dirstate: rust implementation of dirstatemap The `dirstatemap` is one of the last building blocks needed to get to a `dirstate.walk` Rust implementation. Disclaimer: This change is part of a big (10) series of patches, all of which started as one big changeset that took a long time to write. This `dirstatemap` implementation is a compromise in terms of complexity both for me and for the reviewers. I chose to submit this patch right now because while it is not perfect, it works and is simple enough (IMHO) to be reviewed. The Python implementation uses a lot of lazy propertycaches, breaks encapsulation and is used as an iterator in a lot of places, all of which dictated the somewhat unidiomatic patterns in this change. Like written in the comments, rewriting this struct to use the typestate pattern might be a good idea, but this is a good first step. Differential Revision: https://phab.mercurial-scm.org/D6632