This is a sketch of the overall structure of the prior state decoder and
the model type it populates.
Before we can complete this we'll need to slightly rework how the apply
phase emits the raw events that this is consuming, and in particular to
change the raw state representation to be JSON-based to match with how
Terraform Core expects to receive it once reloaded. That will follow in
later commits.
Our model for state for a stack involves a set of objects that is each
identified by an opaque string key. Although those keys are opaque to
callers of Terraform Core, we will actually be using them for some meaning
in Terraform Core itself, since that will avoid redundantly storing the
same information both in the key and in the object associated with the
key.
This therefore aims to encapsulate the generation and parsing of these
keys to help ensure we'll always use them consistently.
