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
Add the side-condition `p > 0` here, to ensure that the below construction for `s` is well-defined. (I am pretty sure that if a CFL has pumping length `p`, then there exists some `p' > p` such that `p'` is also a pumping length for that CFL.)