Move codegen postprocessing to its own module

[pvdrz]

This PR moves all the syn based postprocessing to its own module inside codegen.

[emilio]

This looks great, thanks!

I have one suggestion that avoids some of the macro soup, let me know what you think. I implemented it in 0798bda.

I think it's a bit easier to follow, if you agree feel free to cherry-pick that on top before merging.

Thanks again!

[pvdrz]

I have one suggestion that avoids some of the macro soup, let me know what you think. I implemented it in 0798bda.

@emilio, that's a pretty cool pattern! I'm going to steal it 😬

I did some changes to your commit so PASSES is a const slice so we don't have to update its size when adding a new option. I'm a bit confused about the TODO comment on top of the pass macro. How could you make that as a const fn?

[amanjeev]

This looks lovely!