Improve auto-paragraph to preserve newlines and handle edge-cases better.
This is a very large commit that includes numerous improvements to the
AutoParagraph injector. These are:
* Rewritten flow control of the injector to use almost exclusively
binary conditionals.
* Improved inline documentation with "State" comments, which give concise
examples of what the token stack looks like at flow points.
* Documentation for all flow branches, even those with no actions.
* Factoring out of common operations to improve readability, especially the
new iterator private methods.
* Expanded test-suite which covers new flow points, and corrects some errors
in previous cases.
Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>