Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
My first reaction here was "Can we declare the type of the variable for clarity" but then this requires us to declare the domain first.  This might be another reason to unify this example with the domain set operations text -- declare and name the domains there, then these could be declared as A:[D1] bool = ["a" => ...];   As much as I'm a fan of type inference, I think for a primer like this, being clear is probably the right way to go.  (though I would also be tempted to show a declaration of an associative domain and array without a type declaration to drive home the point that their types can be inferred from their literal values).