Auto merge of #23206 - nagisa:print-io, r=alexcrichton

r? @alexcrichton or @aturon

This still needs to somehow figure out how to avoid unstable warnings arising from the use of unstable functions. I tried to use `#[allow_internal_unstable]` but it still spits out warnings as far as I can see. @huonw (I think you implemented it) does `#[allow_internal_unstable]` not work for some reason or am I using it incorrectly?

diff --cc src/libstd/io/mod.rs
Simple merge

diff --cc src/libstd/io/stdio.rs
Simple merge

diff --cc src/libtest/lib.rs
Simple merge