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 serve
$ hg clone http://localhost:$HGPORT/ copy
abort: * (glob)
[255]
$ test -d copy
[1]
$ "$PYTHON" "$TESTDIR/dumbhttp.py" -p $HGPORT --pid dumb.pid
$ cat dumb.pid >> $DAEMON_PIDS
$ hg clone http://localhost:$HGPORT/foo copy2
abort: HTTP Error 404: * (glob)
[255]
$ killdaemons.py