feat!: cache last tree structure to simplify events

Things like key up/down dont require the items anymore to be used.
Instead a cached last state from last render is used.

examples/example.rs

@@ -142,11 +142,11 @@ fn run_app<B: Backend>(terminal: &mut Terminal<B>, mut app: App) -> std::io::Res
                 KeyCode::Char('\n' | ' ') => app.state.toggle_selected(),
                 KeyCode::Left => app.state.key_left(),
                 KeyCode::Right => app.state.key_right(),
-                KeyCode::Down => app.state.key_down(&app.items),
-                KeyCode::Up => app.state.key_up(&app.items),
+                KeyCode::Down => app.state.key_down(),
+                KeyCode::Up => app.state.key_up(),
                 KeyCode::Esc => app.state.select(Vec::new()),
-                KeyCode::Home => app.state.select_first(&app.items),
-                KeyCode::End => app.state.select_last(&app.items),
+                KeyCode::Home => app.state.select_first(),
+                KeyCode::End => app.state.select_last(),
                 KeyCode::PageDown => app.state.scroll_down(3),
                 KeyCode::PageUp => app.state.scroll_up(3),
                 _ => false,