Log messages generated even by failing parsers

[jgm]

Since we're handling log messages outside of parsec's own state mechanism, we now get log messages even for parsers that fail. This leads to repeated, and in some cases misleading messages.

Example:

% pandoc -f rst
`click here`__

.. _link1: link2_
.. _link2: link1_

__ link1_
^D
WARNING Circular reference 'link1' at line 1 column 15
WARNING Circular reference 'link1' at line 1 column 15
WARNING Circular reference 'link1' at line 1 column 15
<p><a href="">click here</a></p>

Why do we get three messages? Well, first we're parsing the first line looking for a field list, then we're parsing for something else, then a paragraph.

Previously we used updateState from parsec to add messages to state. This worked well, because an update in a parser that fails is rolled back, so we'd only get one message.

A band-aid would be to go through the list and remove duplicates. But better to design so these duplicates don't occur. Especially because in some cases they may be misleading.