WebAssembly support

[jjant]

Description

Split original runty8 crate into many different ones:

• runty8: Main entry point for games created with Runty8.
  Most Runty8 users should probably only depend on this crate.
• runty8-core: Core types needed by every Runty8 related crate
• runty8-runtime: Run standalone Runty8 games in native or wasm targets
• runty8-editor: Run game inside editor
• runty8-winit: Integration layer for winit
• runty8-event-loop: Thin layer over winit/glow/glutin to have a cross-platform OpenGL/WebGL-powered event loop

Move examples to top-level examples directory.

TODO

• Web: Figure out how to get the event loop running at a consistent frame rate.
  It seems that in native we can use ControlFlow::WaitUntil(instant::Instant::now() + instant::Duration::from_millis(33)) , but that's very slow in web, leading to frame times of 34-40ms.
  It looks like we want to use Poll, and handle accumulated times manually. Poll uses requestAnimationFrame.

Edit: Used ControlFlow::Poll everywhere, and fixed my timestep. See d89b7a2.

• Fix mouse move in editor
• Implement setting title functionality inrunty8-event-loop
  Done in 4dfe853.
• Make load_assets! be project-root-based (use CARGO_MANIFEST_DIR)

Not in scope for this PR

• Running the editor in the browser.
• User crates need to add a dependency on include_dir, this is not good but it's probably blocked by include_dir macro cannot be used when re-exported Michael-F-Bryan/include_dir#87