%run-program: Overhaul of asynchronous process spawning
The primary changes made in this branch can be summarised as follows:
- Employ parameter-error and not-implemented-error
- Add and use process-info class with a few corresponding getter functions
- Export functions: process-info-pid, wait-process
- Add functions: launch-program, close-streams, process-alive-p, terminate-process
- Unify exit status codes
- Add tests; unix-specific for now.