dirstate-tree: Make Rust DirstateMap bindings go through a trait object This changeset starts a series that adds an experiment to make status faster by changing the dirstate (first only in memory and later also on disk) to be shaped as a tree matching the directory structure, instead of the current flat collection of entries. The status algorithm can then traverse this tree dirstate at the same time as it traverses the filesystem. We (Octobus) have made prototypes that show promising results but are prone to bitrot. We would like to start upstreaming some experimental Rust code that goes in this direction, but to avoid disrupting users it should only be enabled by some run-time opt-in while keeping the existing dirstate structure and status algorithm as-is. The `DirstateMap` type and `status` function look like the appropriate boundary. This adds a new trait that abstracts everything Python bindings need and makes those bindings go through a `dyn` trait object. Later we?ll have two implementations of this trait, and the same bindings can use either. Differential Revision: https://phab.mercurial-scm.org/D10362