Rewrote LaTeX reader with proper tokenization.

This rewrite is primarily motivated by the need to
get macros working properly (#982, #934, #3779, #3236,
 #1390, #2888, #2118).

We now tokenize the input text, then parse the token stream.
Macros modify the token stream, so they should now be effective in any
context, including math. (Thus, we no longer need the clunky macro
processing capacities of texmath.)

A custom state LaTeXState is used instead of ParserState.
This, plus the tokenization, will require some rewriting
of the exported functions rawLaTeXInline, inlineCommand,
rawLaTeXBlock.

src/Text/Pandoc/Error.hs

@@ -64,6 +64,7 @@ data PandocError = PandocIOError String IOError
                  | PandocTemplateError String
                  | PandocAppError String
                  | PandocEpubSubdirectoryError String
+                 | PandocMacroLoop String
                  deriving (Show, Typeable, Generic)
 
 instance Exception PandocError
@@ -107,6 +108,8 @@ handleError (Left e) =
     PandocAppError s -> err 1 s
     PandocEpubSubdirectoryError s -> err 31 $
       "EPUB subdirectory name '" ++ s ++ "' contains illegal characters"
+    PandocMacroLoop s -> err 91 $
+      "Loop encountered in expanding macro " ++ s
 
 err :: Int -> String -> IO a
 err exitCode msg = do