Use the new print-to-stderr macro when it's stable

[carols10cents]

See rust-lang/rfcs#1869. @zackw has graciously volunteered to help with this!

[zackw]

I should have time this week to read through the current draft and make some notes about what needs changing, which I will post here. I suspect the bulk of the changes will be simply changing println! to somethingprintln! in example code. (Careful reading is needed to figure out which instances of println! should change; it won't be all of them.)

However, based on a very quick skim I think I should warn you that the contents of ch12-06-writing-to-stderr-instead-of-stdout.md (after revision to not talk about writeln!(io::stderr(), ...)) probably need to move into chapter 3.

[zackw]

I finally got around to drafting changes. See PR #615.