Refactor infrastructure for supporting lists

[quephird]

Right now, knowledge and logic of how to access and manipulate lists is spread across LoxList, Interpreter, and NativeFunction. Moreover, there is currently a loophole of sorts which allows instantiation of a List but that doesn't have any of the features of a LoxList, and this is also potentially confusing to users. There ought to be a way to centralize all the business logic for handling of lists, and to close the loophole.

[quephird]

Loophole addressed in #30

[quephird]

I'm gonna close this for now, may reopen later...