latex2unicode: Move sub/super handler into separate function, process_fooscript().
latex2unicode: Generalize find_option() to allow arbitrary expressions.

In extension, I finally came round to introduce a generic stackmachine()
that may be used by all sorts of delimiters.  Modify find_block() to use
it.
t/brightmare.t: Enable pathologic \sqrt test that needs 0ebe08f.