Implement ExtendInto for String, allowing dynamic-length escape replacements

[klardotsh]

Howdy! This is a follow-up PR to resolve the issue I mentioned in a comment earlier this evening: #1206 (comment)

This trait implementation would allow me to stop interning strings (or doing other hackarounds to get all branches of a call to alt to return &str, the only useful thing for my purposes that implements ExtendInto so far) in my string literal parser, which would also let me drop an entire crate worth of dependency!

I hope the included unit test helps illustrate how this could be used - the logic of "escape double quotes, and pass all other escape sequences through to the output" is fairly similar to how Python's raw string literals work, if you're familiar.

Cheers!