Implement pure version of todos example 🎉

The `Widget` trait in `iced_pure` needed to change a bit to make the
implementation of `Element::map` possible.

Specifically, the `children` method has been split into `diff` and
`children_state`.

examples/pure/todos/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "pure_todos"
+version = "0.1.0"
+authors = ["Héctor Ramón Jiménez <hector0193@gmail.com>"]
+edition = "2021"
+publish = false
+
+[dependencies]
+iced = { path = "../../..", features = ["async-std", "debug", "pure"] }
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
+async-std = "1.0"
+directories-next = "2.0"
+
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+web-sys = { version = "0.3", features = ["Window", "Storage"] }
+wasm-timer = "0.2"