u'\\\\\u1234' not fully minified

[gareth-rees]

The string u'\\\\\u1234' could be minified as ur'\\\u1234' for a saving of a character. I prevented this choice because it's absurdly tricky to get backslash-escaping correct in r-prefixed strings.

We mustn't minify u'\\\u1234' as ur'\\u1234' because those two strings are not the same: the first blackslash prevents the second from starting a Unicode escape sequence!

I'm afraid I punted on this one for the moment: at least it currently outputs an accurate result, if not the minimal one.

(Perhaps it would be easier to solve this by generate-and-test than by analysis.)