Update data structures for "slots"

[halfak]

Once https://phabricator.wikimedia.org/T199121 is resolved, the XML dumps will contain a new tag called "slots". That means we'll need to handle "slots".

I think it makes sense to have revision.text point to the "main" slot content. Otherwise, we should have revision.slots.main and revision.slots.<whatever> for other slots.

[amire80]

Is this still relevant? A very naïve grep for slot finds that something related was added in #15, but I might be wrong.