Add memory safe assembly annotations

[nathan-lapinski]

Part of #3336

Adds memory safe annotations for blocks of inline assembly in the following files:

• Address.sol
• Base64.sol
• Clones.sol
• EnumerableSet.sol
• ERC721.sol
• ERC2771Context.sol
• MinimalForwarder.sol
• StorageSlot.sol
• SafeMathMock.sol

As discussed here, I am happy to add more files into this PR before merging, but I wanted to open this now to gather feedback.

This is also my first contribution to this repository, so please let me know if I have made any errors.🙏

PR Checklist

• Tests
• Documentation
• Changelog entry

[frangio]

Thank you @nathan-lapinski!

A few comments:

• PR titles should be written like commit messages, e.g. "Add memory safe annotation", and should not include the issue number
• These annotations aren't really needed in the mock, because it's just for testing purposes I think we should remove it to keep the files as clean as possible.

[nathan-lapinski]

Sounds good. Thank you for the review, @frangio !

I will add a few more files to this PR before requesting another review.

[frangio]

Thanks! This looks good. Do you want to add the remaining files from #3336 (comment) ?

[nathan-lapinski]

Thanks for the review @frangio ! Yes, I’ll go ahead and add the remaining files to this PR.

[nathan-lapinski]

This covers all of the inline assembly usage except for contracts/proxy/Proxy.sol. This usage seems a little more complex, and I can't really tell yet if it's memory safe or not.

If it looks safe, I'd be happy to add it to this PR, or at least add a comment mentioning why it wasn't marked as memory-safe.

[frangio]

Thank you! The Proxy assembly block shouldn't make a difference.