This patch implements the new logic. It passes all of the fstring tests (except of course the ones that had backslashes in the expressions). I'd still like to add some more tests, because I'm not convinced the named unicode escapes are correct (f"\N{LEFT CURLY BRACE}". Because I'm now scanning the decoded utf-8 version of the string, I need to be aware of this syntax and not decide it's part of an expression (because of the braces).