rust-dirstate-status: add `walk_explicit` implementation, use `Matcher` trait This is the first time we actually use the `Matcher` trait, still for a small subset of all matchers defined in Python. While I haven't yet actually measured the performance of this, I have tried to avoid any unnecessary allocations. This forces the use of heavy lifetimes annotations which I am not sure we can simplify, although I would be happy to be proven wrong. Differential Revision: https://phab.mercurial-scm.org/D7529