iced layout markup language & hotswap test

This was created for iced-rs/iced#21

cargo run

todo:

Templated layout engine
- ron-based markup language
- Templates handled with handlebars
Handle user-defined messages for callbacks
- User-defined message type must be serde::Deserialize
User-defined state
Hot-reloading
- Missing a Subscription that watches for file changes
Implement all iced_native widgets
Custom widgets?
fix std::mem::transmute horrors