rust-matchers: make `Matcher` trait object-safe Before this patch, it is not possible to create a `Matcher` trait-object (like `Box<dyn Matcher>`), because of the use of a generic parameters in some methods, namely `impl AsRef<HgPath>`. While this makes the interface less flexible for callers in theory, it does not change anything in the current codebase. Until something like [1] is implemented, this is a "tradeoff" that we need to make anyway. [1] https://internals.rust-lang.org/t/pre-rfc-expand-object-safety/12693 Differential Revision: https://phab.mercurial-scm.org/D9071