Clean up stringify in util/hsc2hs/CrossCodegen

Currently, it's defined

stringify :: String -> String
stringify s = reverse .  dropWhile isSpace . reverse -- drop trailing space
              . dropWhile isSpace                    -- drop leading space
              . compressSpaces                       -- replace each span of
                                                     -- whitespace with a single space
              $ s
    where compressSpaces [] = []
          compressSpaces (a:as) | isSpace a = ' ' : compressSpaces (dropWhile isSpace as)
          compressSpaces (a:as) = a : compressSpaces as

If we're going to go to the trouble of doing this by hand, we might as well take the efficiency advantage doing so can give us:

stringify :: String -> String
-- Spec: stringify = unwords . words
stringify xs = go False (dropWhile isSpace xs)
  where
    go _haveSpace [] = []
    go  haveSpace (x:xs)
      | isSpace x = go True xs
      | otherwise = if haveSpace
                    then ' ' : x : go False xs
                    else x : go False xs

Trac metadata

Trac field	Value
Version	7.9
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Code Coverage
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