rust: samples: add `rust_print` example

Add example to exercise the printing macros (`pr_*!`) introduced
in the previous patches.

Reviewed-by: Finn Behrens <me@kloenk.dev>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
Tested-by: Sergio González Collado <sergio.collado@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>