Dispense of `state` and use plain vars

The `state` object made sense when there were a large number of
stateful variables, but now that there are only three and they
are mostly used for debug, it doesn't matter much anymore. Exposing
them is still interesting (beyond debugging) if one wants to build
more advanced behaviour which requires peeking into rate-limit handling.

Also, change `MegaTwitter` back to just `Twitter`.
It just doesn't look right.