Port parts of JuliaSyntax.jl#329

[c42f]

There's a few things which could be useful to port or revisit from the old prototype in https://github.com/JuliaLang/JuliaSyntax.jl/pull/329. This is a reminder to do that

• [ ] The @ccall benchmark
• [ ] The @__EXTENSIONS__ idea
• [ ] Musings about pattern matching