[ALOY-403] Create Alloy.Collections.instance() and Alloy.Models.instance() functions

Right now the generated code for making sure we have an instance of a singleton Collection (or Model) looks like this:

Alloy.Collections.todo || (Alloy.Collections.todo = Alloy.createCollection("todo"));

It would be nicer to have a function that returns an instance if we need one, or the already created instance if it exists. Basically, the line of code above shouldn't be generated every single time a or is used in markup. This will help reduce the size of the code, and make it more readable.

Alloy.Collections.instance('todo');
Alloy.Models.instance('todo');