Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
This is just a style issue, but I'd be inclined to make this a conditional that throws a halt like "halt("Wait a sec, I thought we had removed Bob!") just to avoid the vagueness implied by assertion failures (IMO).  Or wait, maybe our assert takes a string like this that can be printed out on failure like assert(..., "Wait a second, I thought we had...");  (?)