rust: add `dirstate_tree` module Mercurial needs to represent the filesystem hierarchy on which it operates, for example in the dirstate. Its current on-disk representation is an unsorted, flat structure that gets transformed in the current Rust code into a `HashMap`. This loses the hierarchical information of the dirstate, leading to some unfortunate performance and algorithmic compromises. This module adds an implementation of a radix tree that is specialized for representing the dirstate: its unit is the path component. I have made no efforts to optimize either its memory footprint or its insertion speed: they're pretty bad for now. Following will be a few patches that modify the dirstate.status logic to use that new hierarchical information, fixing issue 6335 in the same swing. Differential Revision: https://phab.mercurial-scm.org/D9085