Generic mutation scheduling system

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

src/systems/scheduler/mod.rs

@@ -0,0 +1,10 @@
+//! A generic utility to sequence long blocking mutations that require a mutable
+//! reference to a shared resource.
+
+mod busy;
+mod system;
+mod canceller;
+mod take_and_drop;
+
+pub use canceller::Canceller;
+pub use system::{SealedOrTaken, SeqScheduler, SharedHandle, SharedState};