matchers: always check the type of new matcher assignments.
matchers: always check the type of new matcher assignments.

* lib/specl/matchers.lua (matchers): Store the actual matcher
functions in a `contents` subtable so that `__newindex` doesn't
ignore reassignments to an existing entry!
(matchers:__index): Automatically reference the contents
subtable.

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

gvvaughan/specl