Reduce lock contention (round 1)

• Use an async (i.e. unlimited buffer) MPSC channel instead of an Arc<Mutex<Vec>> for storing the scored matches in Sorter
• Use Arc<Matcher> instead of Arc<Mutex<Matcher>> for the matcher, as it's not mutated and appears to be threadsafe.

This cuts average iteration time (on the benchmarked machine) from
25.98ms to 16.08ms for the ivy_files benchmark.