Use semantic names for table of contents fragments

[chromakode]

Instead of #toc_0, how about something like #wiki-toc-first-few-words?

[stuartpb]

It looks like this is being done now on the live site (example: https://www.reddit.com/wiki/reddiquette#wiki_in_regard_to_voting), but I can't figure out how based on the code circa html.c line 226, which appears to still use "toc_" + number.

I came looking for the spot where it makes these conversions, as it's kind of broken: it outputs IDs with percent-encoded characters (using a dot instead of a percent) like "wiki_please__don.27t", which, on top of being ugly, poses a problem for CSS selectors.