Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
If you take my advice early on to set associative domains up to be the keys for dictionaries/hash tables, you might draw that back in here.  For example, you could continue the previous statement by saying that where domains provide the keys, arrays provide the values.

I'd use "hash table" rather than "hash" here (because I think of the hash as being the value computed by a hash function, not the array quality.

Really quibbly: Stylistically, I'd use double quotes for English terminology things like this and reserve single quotes for code elements (like identiifers).