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
So I reviewed the reasoning, and I think it all still holds together if you just change the definition of `s` in the manner I suggested above, namely:

```
`s = r#^{p+2}""#^{p+1}"#^{p+2}`

e.g. for `p = 2`: `s = r####""###"####`
```

Then *that* will actually produce an invalid string literal after pumping.