Support for executing a compiled Pyret file from Pyret

- Uses a JS eval; this is bad and needs to be fixed
- Allows us to run Pyret from Pyret
- Added Pause to runtime as a new kind of exception that can asynchronously
  restart the stack
- Fixed a bug in the visitor compiler where module ids could be overwritten
  when new runtimes included them
fixed phase0 built with previous commit