matchers: use util.tostring to display tables for diagnostics.
matchers: use util.tostring to display tables for diagnostics.

* specl/matchers.lua: Import specl.util.
(q): Use util.tostring so that tables output by failed expectations
are legible.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/specl