After studying google closure compiler, I'm leaning towards a multi-ast-pass infrastructure for the minifier.
This is one of the few places where we are going to trade maintainability over performance, given the goal of the minifier is compression size not performance.
All of the terminologies and separation of concerns are aligned with google closure compiler.
Infrastructure of `terser` and `esbuild` are not suitable for us to study nor pursuit. Their code are so tightly coupled - I failed to comprehend any of them every time I try to walk through a piece of optmization. Google closure compiler despite being written in Java, it's actually the most readable minifier out there.
To improve performance between ast passes, I envision a change detection system over a portion of the code.
The benchmark will demonstrate the performance regression of running 5 ast passes instead of 2.
To complete this PR, I need to figure out "fix-point" and order of these ast passes.
We have a strange workaround for `visit_function` where we pass in `ScopeFlags`, to support creating the scope inside `Function`, but setting different flags for `MethodDefinition`s.
Previously `visit_function` took `Option<ScopeFlags>` and then did `flags.unwrap_or(ScopeFlags::empty()) | ScopeFlags::Function` to it. Personally, I found this confusing. When I was looking at `MethodDefinition`, I was wondering "It's a function, why doesn't it set Function flag too?"
This changes makes it more explicit and clear what `ScopeFlags` everything has.
