Please process output in parallel

This bug was originally reported by Mattia Rizzolo (mattia@debian.org) in Debian bug #842837:

Diffoscope is currently single threaded.  Whilst there could be some
external comparators that make use of multiple thread, diffoscope
doesn't really take advantage of multi-threaded/multi-processors
systems.

The work started in the WIP/parallel and WIP/parallel2 branches on the
git repository more then one year ago, but was never completed, possibly
that's a good start point nonetheless.