Further Refine AST

The following changes to the AST would proabaly be nice:

- Add a `Contract` type that bundles all constructors / methods / invariants / storage declarations for a particular contract. It would be nice if this had some constraints on the constructors (e.g. only one passing constructor).
- Add a `System` type that is a collection of related contracts. In the future this could perhaps contain system level invariants

No file or test is named; start by locating the existing AST definitions and representations for constructors, methods, invariants, and storage declarations. Define the intended boundaries and constraints for Contract and System, then confirm that the AST can represent a contract collection and constructor rules without prematurely adding system-level invariants.

Written by the indexing model from the issue text.