Various rustdoc improvements

* Migrate from pandoc to [sundown](https://github.com/vmg/sundown). This takes rendering times of libstd from 30s to 0.5s (no longer a process-per-markdown-block).
* Change code blocks to using sundown syntax instead of pandoc syntax.
* Improve comment stripping in rustdoc (issues mentioned in commit message)
* Add an option to print rustdoc passes (so someone can know what to do with them), and remove the collapse-privacy pass (it's not clear whether the purpose is correct or not).