extra::json and the Rust tokenizer unsoundly cast -1 to char

[thestinger]

This should be fixed ASAP.

[erickt]

Now that std::io is gone, should this be closed?

[alexcrichton]

Aha indeed! Closed by #9901

[thestinger]

It's still an issue though, the title is just wrong. There are a fair bit of these FIXMEs left.

[huonw]

I'm working on this.