Add support for deploy-time linking

[Karrq]

🌟 Feature Request

📝 Description

This features request is to port the existing solc support for deploy-time linking into zksolc. solc achieves this by leaving placeholders in the bytecode to be filled in later (via itself or otherwise).

🤔 Rationale

It allows tooling such as foundry and hardhat to deploy libraries in a temporary environment and link them to the contracts in use, without having to recompile everything, greatly improving UX.

📋 Additional Context

This feature request comes from investigating the following issue on foundry-zksync: https://github.com/matter-labs/foundry-zksync/issues/382

• zksync docs on the non-support of the feature: https://docs.zksync.io/build/developer-reference/ethereum-differences/libraries
• solc docs on the library linking: https://docs.soliditylang.org/en/latest/using-the-compiler.html#library-linking

[hedgar2017]

Hey @Karrq, thank you for this request!

We're currently migrating to our new LLVM-based assembler and linker, and will think how to design this properly on our instruction set.