Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
Aha, array literals!  See previous comment -- I'd introduce these around the same time I introduce arrays.  But note that the example above is closer to what I was calling for just above in terms of being small, arbitrary sets.  I'd do the same with domains before going on to arrays here -- can probably even use the same domains for consistency?