This PR introduces the final major operator: the JOIN operator.
Many things need to be fixed before we can properly support them, and we
handle those. In particular, JOINs always generate qualified column
statements, but we were not handling them correctly at all in the
operators. Not a problem for linear circuits, but fatal for JOINs.
The operator.rs file also becomes incredibly complex with not one, but
two stateful operator. So it is now broken apart.
Closes#3207