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

src/render.rs

@@ -7,7 +7,7 @@ use crate::power::AsMilliwatts;
 use crate::task::Task;
 use crate::time::Periodically;
 use running_average::RealTimeRunningAverage;
-use std::fmt::Debug;
+use core::fmt::Debug;
 
 pub trait HardwarePixel: Send + Sync + Copy + AsMilliwatts + Default + From<Rgb<u8>> + Fract8Ops {}
 impl<T> HardwarePixel for T where T: Send + Sync + Copy + AsMilliwatts + Default + From<Rgb<u8>> + Fract8Ops {}