Tweaks to AMD parsing for interpretations

* "Subject heading" refers to the intro text of a comment
* "heading 22(a)" is now recognized as a header (before it needed to be heading (for|of) 22(a))
* Can refer to "paragraph 22(a)-5"
* Can refer to "entries for 22(a), (b), and (f)(3)" to mean comment headers
* Stray 'and' tokens are compressed
* Restart the "context" every time we enter a new section. In particular, this means that interpretations from previous sections don't make later sections interpretations (as is the case when modifying multiple regs in one notice)