rust-matchers: improve `Matcher` trait ergonomics `VisitChildrenSet` has no need to own the set, this will save allocations. The `file_set` return type change is motivated by both ergonomics and... being able to compile code. The `AlwaysMatcher` does not store a `file_set`, which requires it to return an owned `HashSet`, which in turn would change our return type to `Cow<&HgPath>` (lifetimes omitted). This is both un-ergonomic and troublesome for more complex lifetime issues (especially with the upcoming `FileMatcher` in the following patch). Differential Revision: https://phab.mercurial-scm.org/D7525