Initial Channel implementation

This new type isn't integrated in with any of the existing types yet.
This is a step towards what is needed to implement the command pattern,
which I think is the best approach for #192.

I think we might need a Broadcast channel type as well to allow a
channel type where multiple callbacks can be associated rather than a
single, but it will require a Clone bound or extra allocations and the
current implementation doesn't need either.

src/lib.rs

@@ -20,11 +20,12 @@ mod names;
 #[macro_use]
 pub mod styles;
 mod app;
+mod reactive;
+pub use reactive::{channel, value};
 pub mod debug;
 pub mod fonts;
 mod tick;
 mod tree;
-pub mod value;
 pub mod widget;
 pub mod widgets;
 pub mod window;