Provide implementation of UUPS proxy pattern

[frangio]

EIP 1822 describes the UUPS proxy pattern. This pattern is simpler and should be cheaper as well, so we're interested in providing it as an alternative to the transparent proxy pattern.

[Amxx]

FYI: https://github.com/Amxx/openzeppelin-contracts/tree/feature/UUPS/contracts/proxy/uups (includes tests)

[frangio]

The EIP as published on the eips site is using the "PROXIABLE" storage slot but there is an open PR that changes it to the same slot we use in our upgradeable proxies.

[Amxx]

With that change, it feels like the UpgradeableProxy we currently have is an UUPS proxy. The only missing brick would be an abstract class Proxiable (with a public proxiableUUID(), and an internal _updateCodeAddress that verifies this UUID). What do you think ?

[frangio]

Yeah that sounds good. I don't understand what the UUID is though. I couldn't find an explanation in the EIP.

It seems like it's a mechanism to avoid bricking the upgradeability mechanism. We had some other ideas for such mechanisms and I'm not sure the EIP should be opinionated on which one to implement. I think this is something we should discuss.

[Amxx]

The proxiableUUID() is just a function that returns the slot. It is used to verify that an implementation is "Proxiable". If not, you should not be able to update to it. This is a small security feature.