Add eprint! and eprintln! macros to the prelude.

These are exactly the same as `print!` and `println!` except that
they write to stderr instead of stdout.  Issue #39228.

src/libstd/io/mod.rs

@@ -290,6 +290,8 @@ pub use self::util::{copy, sink, Sink, empty, Empty, repeat, Repeat};
 pub use self::stdio::{stdin, stdout, stderr, _print, Stdin, Stdout, Stderr};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::stdio::{StdoutLock, StderrLock, StdinLock};
+#[unstable(feature = "eprint", issue="39228")]
+pub use self::stdio::_eprint;
 #[unstable(feature = "libstd_io_internals", issue = "0")]
 #[doc(no_inline, hidden)]
 pub use self::stdio::{set_panic, set_print};