Consider merging the registry and the factory

[PaulRBerg]

Context

@razgraf has recently given me feedback on 4.0.0-beta.1. One of his comments touched upon the potentially misleading behavior of the getCurrentProxy function in the situation when the proxy owner transfers ownership via transferOwnership. In this case, getCurrentProxy would return an invalid and misleading address, because the proxy would no longer belong to the provided owner.

It is impossible to prevent this scenario under the current architecture (there is no communication channel between proxies and registry, and, for security reasons, there shouldn't be any).

Proposal

This issue would be eradicated if the registry and the factory contracts were merged into a single contract that wouldn't allow users to deploy multiple proxies. The transferOwnership function in the proxy would be moved to this merged contract.

Pros

• Address the misleading behavior of getCurrentProxy reported by @razgraf
• Lower deployment cost, because fewer contract calls would be chained
• Thinner API, which means fewer ways in which users can use the system in a wrong way (Hyrum's Law)

Cons

• The factory address would have to be set in the proxy constructor, which would complicate the CREATE2 address computation
• Users will only be able to have one proxy at a time, which may not be desirable in some instances, e.g. when debugging. However, there's always the possibility to deploy the proxy manually by running the DeployProxy.s.sol script.

[PaulRBerg]

Locking since I ended up implementing this proposal in #81.