Refactor visitors and transformers

[ishitatsuyuki]

Visitors: switch to a more commonly used model. Projects like serde or rustc are examples.

• This design store data in self. Having a separate context associated type doesn't have much gain. The old context is used for composition instead. With this method, we may add things like per-transformer options in the future.
• We no longer use a register pattern, because it's both coding and codegen overhead, and plus that it interleaves visitor execution.

Transformers: self-explanatory additions to revive the transform(Module) entrypoint. Some tautological tests are removed.

ParentTrackingContext: removed because the usage is unknown.

[ishitatsuyuki]

@cmtt Review ping!

[maciejhirsz]

@ishitatsuyuki hey, sorry for being quiet here, I'll review this whenever I have a moment!

[maciejhirsz]

Btw, given how clean this PR was and how long you waited, if you would like, I'd be more than happy to add you to maintainers!

[ishitatsuyuki]

I would appreciate that. I'm not sure whether I'm motivated now to implement something huge, but given the recent rise in similar interests (e.g. https://github.com/swc-project/swc), I'll try to look what we can share with them.