Suggest eprintln!(…) instead of writeln!(io::stderr(), …)

[killercup]

The error versions of print! and println! have been stable since Rust 1.19. We should suggest using them!

Basically, this:

writeln!(&mut io::stderr(), "foo: {:?}", bar).unwrap();

can be replaced by

eprintln!("foo: {:?}", bar);

We should take care to only suggest this for the default stderr handle, and not, e.g., for one the programmer called .lock() on.

(Inspired by rust-lang/rust#44822)

[ykrivopalov]

Maybe generalize it on both stdout/stderr:
io::stdout().write_fmt(format_args!(...)).unwrap(); => println!(...);
io::stderr().write_fmt(format_args!(...)).unwrap(); => eprintln!(...);

[oli-obk]

Reopening for adding a structured suggestion to the lint in the future