Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
These assignments are interesting because they show that the +=/add methods can take a multiplicity of the index type (which is just an instance of promotion I think?).  I don't think this is obvious -- i.e., it could be that odds is a domain of ranges and you're adding a range to it, so I think I'd call it out as a feature -- maybe it should even be covered earlier when introducing +=/add?

The other thing I'm concerned about with the way that this example is going is that operations like intersection on ranges ought to be computable in O(1) time, so a downside of populating these domains with ranges is that it makes an O(1) operation into an O(n) operation, which may be misleading.  It's a big change from what you'd have, but I'd suggest populating these with other, less regular sets to avoid this spectre.  E.g., make a set of primes and a set of fibs and intersect them (or perhaps just some arbitrary small sets of 3-5 elements where there is some overlap and some not).  You could even continue with your strings domain to avoid switching horses and compute sets like Men v. Women or Adults vs. Kids or such and then intersect those things.  (See possible tie in to array slicing above).