Minimize dependencies for codegen (machine written source files)

[jlapeyre]

Here "codegen" means programmatically written source files.
Codegen scripts are currently run from top level and use cargo test system. Running codegen requires that the entire workspace builds without error.
Can we

• Move them to the syntax crate. We could perhaps do this immediately. Only the syntax crate would need to build without error.
• Move codegen to it's own crate, so that it works regardless of the state of the other crates.