Move `Map` primitives to structures module

[Calvin-L]

The code for heaps is more or less entirely contained in heaps.py. I like how this design worked out, and I think it would be worth moving the code related to maps (er, hash maps, not map-function-over-collection) to a similar maps.py file.

[izgzhen]

i am also thinking about a proposal in which we can have a clean & well-defined interface between the core synthesis engine and plug-in-types.

[Calvin-L]

A starting point you might consider: The _register procedure already allows you to extend Cozy with additional types. The heaps code is connected to the rest of Cozy using that mechanism.

What we are missing is a clean interface; one quick way to make progress would be to create an abstract class with all the methods of the Heaps object and have Heaps and other subclasses extend it.