Related to #63

This PR solves the above issue by fixing a bug in detecting whether a comment is indented or not. When this bug is fixed, fourmolu will always use single line comments for indented haddocks, e.g.

class Foo a where
  -- | asdf
  -- asdf
  foo :: a

{-| asdf
    asdf
-}
bar :: Int

It would be nice if these haddocks could use the same style as non-indented functions, e.g.

class Foo a where
  {-| asdf
     asdf
  -}
  foo :: a

{-| asdf
    asdf
-}
bar :: Int

This seems to mostly be a one-line change:

    getPrinterOpt poHaddockStyle >>= \case
      HaddockSingleLine -> pure True
-     -- Use multiple single-line comments when the whole comment is indented
-     HaddockMultiLine -> maybe False ((> 1) . srcSpanStartCol) <$> getSrcSpan l
+     HaddockMultiLine -> pure False

but this fails when checking if the AST is the same as before, because of whitespace differences in the comment, e.g.

-- from
HsDocString " A multiline\n comment here"

-- to
HsDocString " A multiline\n   comment here\n  "

One possible solution is to also trim leading whitespace when doing the AST check (but not when printing out the haddock in p_hsDocString)