wip: update core for generic Render

crates/tek_core/src/render.rs

@@ -59,3 +59,14 @@ impl<R, T, U> Render<T, U> for RwLock<R> where R: Render<T, U> {
         self.read().unwrap().render(to)
     }
 }
+
+/// Boxed closures can be rendered.
+///
+/// Rendering unboxed closures should also be possible;
+/// but in practice implementing the trait for an unboxed
+/// `Fn` closure causes an impl conflict.
+impl<'a, T, U> Render<T, U> for Box<dyn Fn(&mut T) -> Perhaps<U> + Send + Sync + 'a> {
+    fn render (&self, to: &mut T) -> Perhaps<U> {
+        (*self)(to)
+    }
+}