Make pairs() work with various library objects

Users are reporting disappointment that using pairs on title objects
doesn't let them see the fields available. It's easy enough to add a
__pairs metamethod to allow pairs to work on title objects, so let's do
that.

The same can be done for mw.uri objects.

For mw.message objects, we can easily enough change the implementation
to be like mw.language objects, which doesn't have this problem.

For mw.site.stats, we may as well just remove the load-on-demand feature
since it will be demanded as soon as the environment is cloned for the
first module.

Change-Id: Ie5a3b5684b2bb6c090f9994baa03977687688929