search engine: adding litprog system

I’m adding the “handaxeweb” very primitive literate-programming system
from <https://github.com/kragen/peg-bootstrap> so that I can use it to
automatically extract the source code from the chapter Markdown.
search engine: explaining context of handaxeweb
search engine: handaxewebifying README.md

Now README.md has the extra comments needed to extract an outdated and
somewhat incomplete version of index.py from it.  All that is left is
to update the code in the file and write a build script, and then we
can eliminate the duplication.
search engine: updated code inside chapter text

Now the code extracted from the chapter text isn't missing any code,
although it's in a different order than index.py is, and a few
comments are missing.  I haven't tested it to make sure that it works,
but hopefully it does.
search engine: gitignoring temp files from handaxeweb