Refactor memory data (#836)

This refactors the widget state storage introduced by @optozorax in https://github.com/emilk/egui/pull/257

* Unify the four buckets (`data`, `data_temp`, `id_data` and `id_data_temp`) into a single `data`.
  * Less complexity, and also less chance of error (storing in one bucket, reading from another).
* Store data by `Id` and `TypeId`.
  * Users can thus reuse the same `Id` to store many types.
  * Uses a simple xor of id and typeid, which is fast and good since both id and typeid are already high-entropy hashes.
* Use different suffixes on the functions to pick if you want the data persisted or not (`get_temp`, `insert_persisted`, etc).
  * Writing with one suffix and reading with the other works.
* To store state not bound to a specific `Id` (i.e. only based on type), use the new `Id::null` as the key.

egui_demo_lib/src/apps/demo/password.rs

@@ -27,7 +27,7 @@ pub fn password_ui(ui: &mut egui::Ui, text: &mut String) -> egui::Response {
     // You can read more about available `Memory` functions in the documentation of `egui::Memory`
     // struct and `egui::any` module.
     // You should get state by value, not by reference to avoid borrowing of `Memory`.
-    let mut plaintext = *ui.memory().id_data_temp.get_or_default::<State>(id);
+    let mut plaintext = ui.memory().data.get_temp::<State>(id).unwrap_or_default();
 
     // 4. Process ui, change a local copy of the state
     // We want TextEdit to fill entire space, and have button after that, so in that case we can
@@ -51,7 +51,7 @@ pub fn password_ui(ui: &mut egui::Ui, text: &mut String) -> egui::Response {
     });
 
     // 5. Insert changed state back
-    ui.memory().id_data_temp.insert(id, plaintext);
+    ui.memory().data.insert_temp(id, plaintext);
 
     // All done! Return the interaction response so the user can check what happened
     // (hovered, clicked, …) and maybe show a tooltip: