Dedicated yul builtin to copy all return data and support it in `UnusedStoreEliminator`.

[rodiazet]

Discussed from here

Motivation

Specification

Prepare a Yul builtin according to the spec in the discussion.

Backwards Compatibility

[cameel]

Just to be clear, this is just an idea, we have not decided to implement it yet. It would make it possible to optimize out returndatacopy(), but we also have to consider that against complicating Yul. We could discuss this at one of the design calls, but first we could use some analysis of how often cases where returndatacopy() is unused actually happen. I suspect that we'd only benefit from this after we introduce an IR with memory objects, because otherwise unused memory that we can't optimize out will keep it in place anyway.