adding proof of context-sensitivy of raw string literals

Stumbled across this and thought it would be cool to prove. I've never used Ogden's Lemma before, but I'm pretty sure I used it right. The pumping lemma definitely doesn't seem sufficient for the job. In particular, when using the pumping lemma, you can always just pump one of the quotes, and it's fine. Ogden's Lemma lets you effectively force the pumper to use certain characters in the string.

@cmr
I would prefer if you spelled out the link between these two sentences, namely that the fact that we cannot span *all* internal nodes then implies that `vwx` can only contain external hashes from *at most* one of the left- or right-hand sides (and thus your next sentence follows).

[Of course we still need to fix the overall argument to reflect the actual rules for raw strings, as commented above.]