Generic mutation scheduling system

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

src/utils/boxed_iter.rs

@@ -0,0 +1,27 @@
+/// Utility functions to get rid of tedious explicit casts to
+/// BoxedIter
+use std::iter;
+
+/// A trait object of [Iterator] to be assigned to variables that may be
+/// initialized form multiple iterators of incompatible types
+pub type BoxedIter<'a, T> = Box<dyn Iterator<Item = T> + 'a>;
+/// creates a [BoxedIter] of a single element
+pub fn box_once<'a, T: 'a>(t: T) -> BoxedIter<'a, T> {
+  Box::new(iter::once(t))
+}
+/// creates an empty [BoxedIter]
+pub fn box_empty<'a, T: 'a>() -> BoxedIter<'a, T> {
+  Box::new(iter::empty())
+}
+
+/// Chain various iterators into a [BoxedIter]
+macro_rules! box_chain {
+  ($curr:expr) => {
+    Box::new($curr) as BoxedIter<_>
+  };
+  ($curr:expr, $($rest:expr),*) => {
+    Box::new($curr$(.chain($rest))*) as $crate::utils::boxed_iter::BoxedIter<_>
+  };
+}
+
+pub(crate) use box_chain;