Use namespaced storage for upgradeable contracts

[frangio]

This PR configures the transpiler to use namespaces instead of gaps in the upgradeable code.

There are two changes to the code that I found necessary. In one case, the transpiled code resulted in a stack too deep compilation error, I assume because the $ storage pointer takes up a place in the stack. In another case, I had to change a public immutable variable into a private variable, because immutables without an override get converted to storage variables and the namespace transformation enforces storage variables to be private. This implies that we can't use public immutables in the codebase in general.

The transpiler code can be found in OpenZeppelin/openzeppelin-transpiler#127.

Fixes #2964

[changeset-bot]

🦋 Changeset detected

Latest commit: 8972612

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
openzeppelin-solidity	Major

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[socket-security]

Updated dependencies detected. Learn more about Socket for GitHub ↗︎

Packages	Version	New capabilities	Transitives	Size	Publisher
solidity-ast	0.4.49...0.4.50	None	+1/-0	264 kB	frangio

[frangio]

I uploaded the result of this transpilation to a branch in openzeppelin-contracts-upgradeable so we can review the resulting changes: OpenZeppelin/openzeppelin-contracts-upgradeable@d16ec78

[frangio]

The latest results are in OpenZeppelin/openzeppelin-contracts-upgradeable@3b8d9b1. What changed was the location of initializer functions, and the latest updates to zero out the least significant byte in the location.

[mudgen]

With this change OpenZeppelin upgradeable contracts should now be compatible with EIP-2535 Diamonds. Would be great to see an implementation of it in OpenZeppelin.