Muse reader: avoid reparsing at the top level

Blocks following paragraphs are parsed only once at the top level.
Lists still take exponential time to parse, but this time is not
doubled anymore when this list terminates paragraph.

src/Text/Pandoc/Readers/Muse.hs

@@ -130,8 +130,7 @@ instance HasLogMessages MuseState where
 parseMuse :: PandocMonad m => MuseParser m Pandoc
 parseMuse = do
   many directive
-  blocks <- mconcat <$> many parseBlock
-  eof
+  blocks <- parseBlocks
   st <- getState
   let doc = runF (do Pandoc _ bs <- B.doc <$> blocks
                      meta <- museMeta st
@@ -250,6 +249,19 @@ directive = do
 -- block parsers
 --
 
+parseBlocks :: PandocMonad m
+            => MuseParser m (F Blocks)
+parseBlocks =
+  try (mempty <$ eof) <|>
+  try blockStart <|>
+  try paraStart
+  where
+    blockStart = do first <- blockElements
+                    rest <- parseBlocks
+                    return $ first B.<> rest
+    paraStart = do (first, rest) <- paraUntil ((mempty <$ eof) <|> (blankline >> blockStart))
+                   return $ first B.<> rest
+
 parseBlock :: PandocMonad m => MuseParser m (F Blocks)
 parseBlock = do
   res <- blockElements <|> para