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
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