New API

WIP

* [ ] Fully separate concerns between Scenario and Runner
** [ ] Runner should take a pre-built Scenario
** [ ] Scenario `#compile` should be renamed `#export` and take a path at this point
* [ ] Overhaul CLI
* [ ] Full API documentation
* [ ] Allow running a scenario without dumping it to disk
* [ ] Allow execution of existing scenarios via a manifest
* [ ] Other stuff?