src: use core instead of std in some places, aiming for no_std compat

src/task.rs

@@ -1,6 +1,6 @@
-use std::fmt;
+use core::fmt;
 
-pub trait Task: std::fmt::Debug {
+pub trait Task: core::fmt::Debug {
     fn tick(&mut self) {}
     fn start(&mut self) {}
     fn stop(&mut self) {}