Support hot-reload preview

[AzurIce]

The main purpose is to offer more convinience experience when writing anim, and attempt to reduce user's compile time.

User will write their Scene code in a lib crate with dylib crate-type.

Thus, the meta info of scene and the preview/render target all needs to be done in "lib" instead of "main".

So, we can use linkme, and wrap it into a ranim's macro for preview and render.

Then, defining a scene to be previewed/rendered will be like this:

#[preview]
#[render(name = "output", fps = 60]
fn arc() -> Box<dyn SceneConstructor> {
    Box::new(ArcScene)
}

This introduces a new crate ranim-cli:

• ranim preview: launch a preview app, watch the lib crate and rebuild it to dylib when it is changed, then reload it and applies to the preview app.
• ranim render: build the lib crate and load it, then render it to video.

And there may be a complete re-organization of crates layout in the near future.