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
Note that fetching any title besides the one for the current page is
considered "expensive". It also records the title fetched in the
ParserOutput so it will be listed in pagelinks, just like #ifexists.
This also moves the ToString test formatter into TestFramework.lua, so
TitleLibraryTests.lua can use it too.
Change-Id: I799f3289a37fe1349b6bca5758829acf82cb718f
