rust-matchers: remove default implementations for `Matcher` trait We don't expect a whole lot of matchers to be defined, and this makes it more obvious what a matcher does by reading its `impl Matcher for FooMatcher`. This patch has the added benefit of fixing the `AlwaysMatcher`, its `matches` function differs from the former default. Differential Revision: https://phab.mercurial-scm.org/D7255