Clean up RunScript helpers and Evaluator code

[lihaoyi]

Not sure exactly how, but they definitely feel a bit out of place, as a bunch of static helpers wrapping Evaluator

Noticed when working on #4091

Would need to be done in 0.13.0 when we can break bincompat

[lihaoyi]

#4144 helps a bit, but more to be done. In particular we can get rid of the Terminal.Task case class and branches since outside of test suites nobody should be running anonymous tasks directly