Change ``setimmutable`` Yul builtin to reference code object, or use special ``codecopy`` version that does the replacement..

Came up in https://github.com/ethereum/solidity/pull/8583#discussion_r419307365

It might be nice to generalize ``setimmutable``, resp. reduce the assumptions it makes. Two options came up so far:

Use additional arguments for the memory offsets of the code to-be-modified and the name of the code object in ``setimmutable``:
```
setimmutable(0 /* memory offset of code to be modified */, "", "", )
```
Or introduce a new variant of ``codecopy`` that implicitly replaces the immutables - however I'm not sure how this would specify the immutable values - would it take a long list of arbitrary arguments? Would it specify a memory area and an order in memory?

Start with the discussion linked from PR #8583, then compare the two proposed designs for generalizing setimmutable or introducing a special codecopy variant. The issue does not name files or tests; done would require an agreed design that specifies how the code object and immutable values are referenced.

Written by the indexing model from the issue text.