Tracking issue for addition of eprint/eprintln macros

[alexcrichton]

Tracking issue for for rust-lang/rfcs#1869

@zackw would you be interested in sending a PR?

[zackw]

>.> Reopen #39229 and proceed from there?

[alexcrichton]

Sure yeah, want to resubmit that as a new PR?

[zackw]

Is it actually necessary to resubmit? Couldn't you just reopen the existing one?

[alexcrichton]

Let's stick with a new PR, easier on bors in general

[steveklabnik]

@zackw when you make your new PR, mind also adding an entry to https://github.com/rust-lang/rust/tree/master/src/doc/unstable-book ? It doesn't need to actually have docs in it. Check out any of the chapters for the little template.