Add graph validation/compilation

We should be able to validate the DAG. Specific things we can check for:
1. For all nodes that read state item X, given an initial state, does there exist a path that does not provide the required state?
2. For all produced keys, do types match up? (when we have typing)?
3. Is there a path that will not end up in a valid state (E.G. nowhere to go)?

Then on execution, we should be able to say:
1. If there is a `halt_after`, is there a path that will reach it?
2. If there is a `halt_before`, is there a path that will reach it?

Idea for (1):

```python
app = ....build(validate=True) # on build, we can validate
app = ....build(validate_types=True, validate_paths=True) # if you want to do different things
```

Idea for (2)

```python
app = ....build()
app.run(halt_after=..., validate=True)
```

Note this is not friendly towards state with default values as we don't expose that in the framework... But I think types + defaults should be easy enough to expose.