Implemented unindent a little differently, hopefully simpler.

I've given this another try. I've tried to simplify the semantics for how the beginning and end are treated a bit. I've also tried to explain it in the comment.

I've also switched back to `>>>`, I like that better. But I'm not passionate about it.

@sol: I wonder about your doctest comment. In my environment the doctest wasn't executed. I've fixed that, now it is being executed. And haddock looks good, too. But...am I missing something?

(And I've removed the code for `normalizeLines`.)