eprint! and eprintln! in Rust 1.19

[jan-schreib]

Since Rust 1.19 was released there are the eprint! and eprintln! macros to write to stderr.
Chapter 12.6 in the book can be updated.
I think it should be added to the appendix. The new break loop feature is also in there.

rust-lang/rust#41192

Thx and please keep up the great work!

[steveklabnik]

eprintln! was already added, in #811

[carols10cents]

Yes, rather than add eprintln! to the "newest features" appendix, we decided to incorporate it into the book, mostly because it makes the book nicer!! And we haven't frozen the relevant chapters yet :)

As steve mentioned, we've already incorporated it into Chapter 12. The trick is that the book rides the trains now, but it's a train behind. The changes to chapter 12 made it into beta, so they're currently viewable at https://doc.rust-lang.org/beta/book/second-edition/ch12-06-writing-to-stderr-instead-of-stdout.html and they'll make it into stable with the 1.20 release.

Thank you!