AST improvements to support conversion to rustc AST

[luca-barbieri]

This is a set of improvements that I done while creating a rustc branch that can use rust-analyzer to parse code and convert the rust-analyzer AST to rustc's internal AST format, found at rust-lang/rust#70745

The goal is to extend the helper classes implementing AstNode and similar classes so that the AST can be examined solely using them.

[bjorn3]

The last four commits are committed by Your Name.

[matklad]

I'll try to merge most of these during the next few days! I'd rather do this myself manually, as this'll conflict with everything!

[matklad]

I've merged most of the things here into master. Most notable things not merged:

• the AstNode and AstToken traits are left as is. No new AstElement traits is added. I would like to separatenly consider the most perfect interface here
• no support for #[sep]: this needs to be examined on a case-by-case basis, but my hunch is that we actually just want to add some extra wrappers to the AST. Like, the problem with pat macro_call being confused with an expr macro call was solved by introduction of ast::MacroPat

Thanks for putting this together @luca-barbieri !