Org reader: replace ugly code with view pattern

Some less-than-smart code required a pragma switching of overlapping
pattern warnings in order to compile seamlessly.  Using view patterns
makes the code easier to read and also doesn't require overlapping
pattern checks to be disabled.

src/Text/Pandoc/Readers/Org/Blocks.hs

@@ -1,6 +1,6 @@
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE RecordWildCards #-}
-{-# OPTIONS_GHC -fno-warn-overlapping-patterns #-}
+{-# LANGUAGE ViewPatterns #-}
 {-
 Copyright (C) 2014-2016 Albert Krewinkel <tarleb+pandoc@moltkeplatz.de>
 
@@ -54,7 +54,6 @@ import           Data.Char ( isSpace, toLower, toUpper)
 import           Data.List ( foldl', intersperse, isPrefixOf )
 import qualified Data.Map as M
 import           Data.Maybe ( fromMaybe, isNothing )
-import qualified Data.Sequence as S
 import           Network.HTTP ( urlEncode )
 
 --
@@ -142,7 +141,7 @@ headlineToBlocks hdln@(Headline {..}) = do
     _ | any isArchiveTag  headlineTags     -> archivedHeadlineToBlocks hdln
     _ | isCommentTitle headlineText        -> return mempty
     _ | headlineLevel >= maxHeadlineLevels -> headlineToHeaderWithList hdln
-    _                                      -> headlineToHeaderWithContents hdln
+    _ | otherwise                          -> headlineToHeaderWithContents hdln
 
 isNoExportTag :: Tag -> Bool
 isNoExportTag = (== toTag "noexport")
@@ -154,8 +153,8 @@ isArchiveTag = (== toTag "ARCHIVE")
 -- FIXME: This accesses builder internals not intended for use in situations
 -- like these.  Replace once keyword parsing is supported.
 isCommentTitle :: Inlines -> Bool
-isCommentTitle xs = (B.Many . S.take 1 . B.unMany) xs == B.str "COMMENT"
-isCommentTitle _  = False
+isCommentTitle (B.toList -> (Str "COMMENT":_)) = True
+isCommentTitle _                               = False
 
 archivedHeadlineToBlocks :: Headline -> OrgParser Blocks
 archivedHeadlineToBlocks hdln = do