This adds test properties for the various laws that the instances of
Result should follow; I could not find (offline) laws about `mappend',
but otherwise I implemented all laws that I could find.

Note that we have to silence hlint warnings for the things we want to
test, as otherwise hlint is all "this is already true based on the
functor law, why 'fmap id' and not just 'id'?".

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>