Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
Should probably call out the sorted() capability here and maybe comment again on the fact that associative domains/arrays are printed out in a nondeterministic order otherwise.  I'd pretty much forgotten about this capability.  I wonder if we'd want to replace this with the more general sorterator...  I wonder if there's any downside.