Migrate Proxy contract

[martriay]

[martriay]

This issue is currently stalled due lack of a fallback function mechanism -- key to implement any kind of proxy.

This is because in Starknet two of the main use cases of proxies are covered by other mechanisms:

• upgrades are achieved through the replace_class syscall
• clones are how most contracts already work: contracts are instances of a class hash

I think there's still arguments to have proxies in Starknet. Beacon Proxies are an example, but novel use cases may also appear.

In any case, we can't move forward with this issue until fallback functions are supported in Starknet.

[ericnordelo]

Closing because we won't have a callback function. I still think at least Beacon Proxies might be helpful. We will open an issue for it if we find demand.