Proposed feature: IdentityMap pattern

More on this feature in the Rails API documentation: http://api.rubyonrails.org/classes/ActiveRecord/IdentityMap.html

And more on the pattern here: http://www.martinfowler.com/eaaCatalog/identityMap.html

I would like to hear if there is any interest in such a feature.

_NB. This PR is only a test for this feature, which is of course failing._