Implements a basic sys/exec for testing

[alaviss]

The current version in features/exec-prototype branch is the API we eventually want, but due to the sheer size it's hard to write all the tests in one go.

So we are gonna pick small parts out and start implementing and testing those instead. Current checklist:

• findExe()
• Launcher
  • program
  • args
  • launch()
• Process
  • wait()
• ProcessStatus

The Windows version will be implemented as part of this.

[alaviss]

Needs #1