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)