Generic mutation scheduling system

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

src/systems/asynch/mod.rs

@@ -1,5 +1,9 @@
-mod system;
-mod types;
+//! An event queue other systems can use to trigger events on the main
+//! interpreter thread. These events are handled when the Orchid code returns
+//! `system::async::yield`, and may cause additional Orchid code to be executed
+//! beyond being general Rust functions.
+//! It also exposes timers.
 
-pub use system::{AsynchConfig, InfiniteBlock};
-pub use types::{Asynch, MessagePort};
+mod system;
+
+pub use system::{AsynchSystem, InfiniteBlock, MessagePort};