"Step back" feature in single stepper

The new graphical single stepper in Factor 0.84 can now travel back in time and "step back" through a quotation. This only restores the stack contents, it does not undo side effects. Still, even though the time travel is far from perfect, it is still useful, especially for beginners learning the language and figuring out how stuff works. They can step over some words, then rewind and step in to look at things in more detail.