lib: rust: Add simple printk support to Rust

When `CONFIG_PRINTK` is enabled, provide macros `printk` and `printkln`
within the `zephyr` crate that applications can use to print.  This
currently sends messages, with a small amount of buffering, using
`k_str_out`.

Until C functions are made generally available from the Rust side,
we use a small wrapper function to call `k_str_out`, as it is
implemented as an inline syscall.

Signed-off-by: David Brown <david.brown@linaro.org>