Generic mutation scheduling system

IO adapted to use it
Also, Atoms can now dispatch type-erased requests

src/systems/scheduler/canceller.rs

@@ -0,0 +1,32 @@
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
+
+use crate::atomic_inert;
+
+/// A single-fire thread-safe boolean flag with relaxed ordering
+#[derive(Debug, Clone)]
+pub struct Canceller(Arc<AtomicBool>);
+atomic_inert!(Canceller, typestr = "a canceller");
+
+impl Canceller {
+  /// Create a new canceller
+  pub fn new() -> Self {
+    Canceller(Arc::new(AtomicBool::new(false)))
+  }
+
+  /// Check whether the operation has been cancelled
+  pub fn is_cancelled(&self) -> bool {
+    self.0.load(Ordering::Relaxed)
+  }
+
+  /// Cancel the operation
+  pub fn cancel(&self) {
+    self.0.store(true, Ordering::Relaxed)
+  }
+}
+
+impl Default for Canceller {
+  fn default() -> Self {
+    Self::new()
+  }
+}