Better user programs with a special std. Sleep and exit are calling scheduler instead of wfi.

src/io.rs

@@ -7,7 +7,7 @@ use log::{LevelFilter, SetLoggerError};
 
 use crate::uart::write_uart;
 
-fn print(content: String) {
+pub(crate) fn print(content: String) {
     write_uart(content);
 }