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
Strictly speaking, I do not believe it is possible to have such a small pumping length for any non-trivial language, but sure, I'll toss in the constraint for clarity. Of course, even if `p` was 0, we use `p` as an exponent in our construction only because it is neat and minimal. We could easily use `p+100` in our proof and nothing would change.