more ReprTree refactoring

- <Vec Char>  ~>  <Vec ReprTree>  packing/unpacking
- remove Arc<RwLock<>> around EditTree
- gentle ListEditor update
- reactivate PosInt example

examples/tty-02-digit/src/main.rs

@@ -65,7 +65,7 @@ async fn main() {
         let ctx = ctx.clone();
         let digit_edittree = rt_digit.edittree( &ctx );
         move |ev| {
-            digit_edittree.get().write().unwrap().send_cmd_obj(ev.to_repr_tree(&ctx));
+            digit_edittree.get_mut().send_cmd_obj(ev.to_repr_tree(&ctx));
         }
     });
 
@@ -91,7 +91,7 @@ async fn main() {
             .offset(Vector2::new(1,1)));
 
         comp.push(rt_digit
-            .edittree( &ctx ).get().read().unwrap()
+            .edittree( &ctx ).get()
             .display_view()
             .offset(Vector2::new(3,2)));