Add showMultiLineString to GHC.Show
This is part of the fix for #4436
showMultiLineString :: String -> [String]
-- | Like 'showLitString' (expand escape characters using Haskell
-- escape conventions), but
-- * break the string into multiple lines
-- * wrap the entire thing in double quotes
-- Example: @breakMultiLineString "hello\ngoodbye\nblah"@
-- returns @["\"hello\\", "\\goodbye\\", "\\blah\"" ]@
-- where those "\\" are really just a single backslash
-- (but I'm writing them here as Haskell literals)