Add loop function from MVC

[bosu]

• It has no dependencies other than pipes ones.
• It is useful on its own, without regard to MVC package.

The only downside I can think of is loop being a very common name (e.g. first patch in the series). OTOH, Pipes.Prelude has other common names (e.g. map) and requires careful imports even now.

[Gabriella439]

Yeah, I'd love to add this with two small modifications:

• Rename the clashing helper functions to go instead of loop
• Remove the long helper comment, mainly because a lot of it only makes sense for the mvc library

[bosu]

The rename is done.

IMHO, the long comment is pure gold :) I was unable to delete it, hence moved it into ListT section of the tutorial module.

If you still think we ought to delete it, I'll unhappily oblige :)

[Gabriella439]

Alright, we can keep it, but it needs a few more edits, since some of the stuff in there is mvc-specific (like the references to "model" or the base StateT monad). I will go through the commit and point out the parts that need to be slightly changed.

[bosu]

Done.

[Gabriella439]

Thanks! I really appreciate the time you took to transplant this. :)