Yul codegen for immutables.

[chriseth]

Part of #8551

[chriseth]

Still needs tests to be activated.

[chriseth]

Should be complete now.

[cameel]

I don't feel confident enough about this code to say that there isn't anything missing (hence no accept) but I carefully read through it all and did not notice anything that would be wrong in an obvious way. Just one stylistic remark about resetContext().

[ekpyron]

Looks good, but some comments and remarks.
I'd be fine with merging as is, though, so I'm already approving.
Also: what's the plan for this for wasm? Actually not sure at the moment what state the wasm codegen is in in general.

[ekpyron]

I'm still wondering whether this should at some point have a much more general signature, like

setimmutable(0 /* memory offset of code to be modified */, "<object>", "<immutableName>", <var>)

Then immutable references would be an implicit property of yul objects and the semantics would be clearer even if e.g. there's no unique runtime object or a deploy routine that doesn't write code to zero in memory (although I'd not be sure about details, like if the memory offset should be a literal, etc.)...
But then again we might come up with an even better way in the future and I guess it's fine for now (especially since the assembly wouldn't support the extended signature right away).

[chriseth]

Referencing the object sounds like a good idea!

[chriseth]

Maybe we could come up with a more sophisticated codecopy function that performs the code copy and the immutable replacement at the same time.

[ekpyron]

I'll create an issue for it.