Make it easier to define high-level ASTs?

[oxalica]

Currently we provides traits and auxiliary functions in rowan::ast, but it's still difficult to use.

• In the example, we define ASTs using a custom macro. https://github.com/rust-analyzer/rowan/blob/6a94212a4954e5a0dbc5986544cfcfe0e5de9eda/examples/s_expressions.rs#L268-L270
• In rust-analyzer, we even use code generation to produce a 4.8k lines file crates/syntax/src/ast/generated/nodes.rs.

Could we make it easier to define ASTs and child extractors? Maybe using some DSL with proc-macros or build.rs for generation?