Real pages render three to twelve times faster

This release is almost entirely speed, and every single change was verified to produce pixel-for-pixel identical pages. We kept profiling real websites instead of guessing, and the profiles kept naming things the engine was doing over and over for no reason: measuring the same piece of text once for every ancestor that asked, building a short string on every single style lookup, copying a page's entire set of design variables into every element on it, and testing thousands of style rules against elements they could never possibly apply to. Fixing those, an encyclopedia article now takes 1.1 seconds to style and lay out instead of 3.8, a Q&A site question 1.4 instead of 4.3, and a code-hosting site repository page 0.12 instead of 1.5. Two visible faithfulness fixes came out of the same work: the encyclopedia's language button showed only the number and now reads "277 languages", and encyclopedia infoboxes are sized and wrapped exactly the way the reference browser sizes and wraps them.