Generic mutation scheduling system

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

src/systems/mod.rs

@@ -1,13 +1,13 @@
 //! Constants exposed to usercode by the interpreter
 mod assertion_error;
-mod asynch;
+pub mod asynch;
 pub mod cast_exprinst;
 pub mod codegen;
-mod io;
+pub mod io;
 mod runtime_error;
 pub mod stl;
+mod directfs;
+pub mod scheduler;
 
 pub use assertion_error::AssertionError;
-pub use asynch::{AsynchConfig, InfiniteBlock, MessagePort};
-pub use io::{io_system, IOStream, IOSystem};
 pub use runtime_error::RuntimeError;