Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
I would add "They are most commonly used to represent arbitrary sets, or to represent the set of keys for a dictionary/hash table."  (in which case I might also drop 'arbitrary' from your sentence to avoid repetition).

I don't know how much to be truthful to the current implementation vs. the intention -- they don't support any index type presently (I believe, primarily, because we haven't defined hash tables for all types).  This feels like an unfortunate thing to bring up so early, though.  We could say "are designed to represent" as a way of creating wiggle space for ourselves.