Refactor memory data (#836)

This refactors the widget state storage introduced by @optozorax in #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.

Author: emilk

CHANGELOG.md

@@ -10,6 +10,9 @@ NOTE: [`epaint`](epaint/CHANGELOG.md), [`eframe`](eframe/CHANGELOG.md), [`egui_w
 ### Added ⭐
 * Add context menus: See `Ui::menu_button` and `Response::context_menu` ([#543](https://github.com/emilk/egui/pull/543)).
 
+### Changed 🔧
+* Unified the four `Memory` data buckts (`data`, `data_temp`, `id_data` and `id_data_temp`) into a single `Memory::data`, with a new interface ([#836](https://github.com/emilk/egui/pull/836)).
+
 
 ## 0.15.0 - 2021-10-24 - Syntax highlighting and hscroll