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

src/platform/smart_leds_lib.rs

@@ -6,7 +6,7 @@ use crate::power::brightness_for_mw;
 use crate::geometry::*;
 use crate::mappings::*;
 
-use std::fmt::Debug;
+use core::fmt::Debug;
 
 struct StrideSampler<'a, P: Pixbuf> {
     pixbuf: &'a mut P,
@@ -26,7 +26,7 @@ impl<'a, P: Pixbuf> PixelView for StrideSampler<'a, P> {
 }
 
 impl<P: Pixbuf, T: FastWrite> Debug for StrideOutput<P, T> {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         f.debug_struct("StrideOutput").finish()
     }
 }