Be smarter about string escaping

[Eijebong]

Something like

proc_macro2::Literal::string(&format!("Couldn't find {}", class_name));

will end up as "Couldn\'t find SomeProtocol" when used in quote!
Since we're in a string surrounded by double quotes, the escaping there is useless.

[dtolnay]

Thanks! I would accept a PR to fix this.