This change ensures that spans are contiguous and that all source is part
of a span. This means that a character can't be 'lost' and not a member of
any span.
And guess what? We have a bug like that. So now a few tests are skipped
due to that bug.
Also made some changes to tests that construct invalid spans or spans
without correct locations as their expected input. This allows us to add
the above verification to all parser tests.
