Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
In this next section, I think you're getting ahead of yourself.  Shouldn't we first show how array elements can be assigned for existing indices?  (i.e., Scores is still all 0's at this point, isn't it?  How about starting by giving some people some scores.  You could also do something like initialize everyone's score to 100 and then spot-replace a few scores with other values to demonstrate that "traditional array operations are supported for associative arrays like whole-array assignment and indexing:"

Scores = 100;
Scores["Robert"] = 95;
Scores["Alive"] = 97;

As I type this, I'm having the typical "should array names be singular or plural" conundrum...