Performance Tweaks

1. Only cache interp children that are titles. This will exclude structural nodes (which exist purely to match the structure of the regtext) and interp paragraphs (i.e. everything after the dash in 111.22(c)(4)-5.vi.G). Not a huge win, but helps
2. Huge win: Instead of generating a new set of layer appliers (and thereby hitting the layer cache) for each slide-down interpretation, generate them once for the interpretations of a whole section. This dramatically reduces the page load time for pages with lots of interpretations (e.g. for 75 interpretations, 85% reduction in response time)