Add forUntil family

[chris-martin]

These loops specifically address the pattern seen in Data.ByteString.hGet and Data.Text.IO.hGetChunk - these functions read a chunk of input from a handle and return an empty string when the end of the input is readed.

For example, a simple "cat" program might look like this:

import Control.Monad.Loops
import System.IO
import qualified Data.Text as T
import qualified Data.Text.IO as T

main = 
main =
  do
    [path] <- getArgs
    withFile path ReadMode $ \h ->
        forUntil_ (T.hGetChunk h) T.null $ \chunk ->
            T.putStr chunk