API: Rename AstContext -> MarkerContext (Let's break everything 💥)

[xFrednet]

This PR has two gigantic breaking changes:

• AstContext was renamed to MarkerContext (See [Discussion] Rename AstContext -> MarkerContext or something similar? #243)
• Span and related structs have been moved to the new marker_api::span module

If anyone uses Marker right now, they will definitely notice this change. But I believe this is worth it and still acceptable at this point.

[xFrednet]

I've done a small update of Marker's readme, I hope this won't conflict with #255

@Veetaha I've stumbled across the ## Contributing section again during this update. In this regard, I wanted to ask if you remember running into any trouble when you got started with contributing?

[Veetaha]

The only trouble that I had was figuring out that I need to add this to the RA vscode config to make it resolve the private rustc crates

{
    "rust-analyzer.rustc.source": "discover"
}

[xFrednet]

Nice catch! I've updated the documentation (What a productive way to procrastinate on some other things I should actually work on ^^)