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 wonder if these comments/pointers should be moved further up the file to where sparse and opaque are pointed out and unified.  I.e., I'm imagining a little directory that says that this file focuses on the abstract concept of domains primarily within the context of rectangular domains; that arrays.chpl focuses on arrays overall and rectangular arrays (if true); that associative domains and arrays (for sets/hash tables) are in associative.chpl; that sparse domains/arrays (for sparse data structures) are handled in sparse.chpl; that opaque domains/arrays (for unstructured or pointer-based data structures) are handled in opaque.chpl.

Is it accurate that nothing needs to be changed/removed from arrays.chpl?