task: move Scheduler into a trait, define static-sized scheduler implementation, expose as a system task runner in platform

src/platform/mod.rs

@@ -9,14 +9,15 @@ pub mod esp32;
 pub type DefaultBoard = esp32::Esp32Board;
 
 use crate::render::{Output, Surfaces};
-use crate::task::Task;
+use crate::task::Scheduler;
 
 pub trait Board {
     type Output: Output;
     type Surfaces: Surfaces;
+    type Scheduler: Scheduler;
 
     fn take() -> Self;
     fn output(&mut self) -> Self::Output;
     fn surfaces(&mut self) -> Self::Surfaces;
-    fn system_tasks(&mut self) -> Vec<Box<dyn Task>>;
+    fn system_tasks(&mut self) -> Self::Scheduler;
 }