Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
"domains of integers" makes me think -- could we do something closer to the top of this file to cover different index types for associative domains?  It could simply be a statement in comments, or we could have some examples of different domain types just to show the flexibility.  It feels like some key cases to cover in addition to strings and integers might be reals (because they're more interesting indices than integers), classes and records (to emphasize that the former hashes on identity and the latter on value), and then allude to "or any other type."  This might also be a good place to indicate that the current implementation may not support all types (not sure how well we can characterize what is or is not supported -- I know Vass has tried to enumerate these cases before and maybe has filed futures against it?).  Having said all that, I'm not precisely sure where to put it.  If it's just a comment, I think it could go at the top; if it's backed by code (and it need not be, IMO, though I do think it would drive some of the points home such as classes vs. records) then I'm worried that putting it anywhere before here would delay key features too much.  So maybe we should start with a comment at the top of the file in any case and then consider adding code towards the end of the file to illustrate (or not adding code).