Expose iced_pure through a pure feature in iced

Besides exposing the `iced_pure` crate, enabling the `pure` feature also
provides pure versions of both the `Application` and `Sandbox` traits!
🎉

src/pure.rs

@@ -0,0 +1,29 @@
+//! Leverage pure, virtual widgets in your application.
+//!
+//! The widgets found in this module are completely stateless versions of
+//! [the original widgets].
+//!
+//! Effectively, this means that, as a user of the library, you do not need to
+//! keep track of the local state of each widget (e.g. [`button::State`]).
+//! Instead, the runtime will keep track of everything for you!
+//!
+//! You can embed pure widgets anywhere in your [impure `Application`] using the
+//! [`Pure`] widget and some [`State`].
+//!
+//! In case you want to only use pure widgets in your application, this module
+//! offers an alternate [`Application`] trait with a completely pure `view`
+//! method.
+//!
+//! [the original widgets]: crate::widget
+//! [`button::State`]: crate::widget::button::State
+//! [impure `Application`]: crate::Application
+pub use iced_pure::{Element as _, *};
+
+/// A generic, pure [`Widget`].
+pub type Element<Message> = iced_pure::Element<Message, crate::Renderer>;
+
+mod application;
+mod sandbox;
+
+pub use application::Application;
+pub use sandbox::Sandbox;