Interpreter sandbox

[mergeconflict]

Users shouldn't be allowed to troll like:

System.exit(0)

or

while (true) println("LOL")

or

val f = new File("/")
f.delete()

I think we should be able to use levee (forked to GitHub here) for this...

[mergeconflict]

@andrewnguyen I think we're getting close. We shouldn't load levee-instrumentor as an agent. Instead I think we should override IMain's classloader, and call into levee-instrumentor there.