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
#require black
$ cd $RUNTESTDIR/..
$ black --config=black.toml --check --diff `hg files 'set:**.py - mercurial/thirdparty/** - "contrib/python-zstandard/**"'`