Refactor access to Checkpoint struct without using memory

[0xVolosnikov]

Fixes mentioned in:
#4495 (comment)

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[0xVolosnikov]

I would also like in this PR to slightly change some lines in:
https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/structs/Checkpoints.sol

Since in this contract the structure (-s) is repeatedly written to the memory. Also, I suspect that such structure reading can create an overhead (since self is read multiple times).

However, this may require changing relatively many lines of code, and I'm not sure that such changes are desired for version 5.0. So I would like to hear the opinion of the maintainers on this. I'm not sure if it's worth creating a separate issue for this, but if necessary, I can create one.

[frangio]

Looks good, gas is slightly impproved.

[frangio]

I would also like in this PR to slightly change some lines in: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/structs/Checkpoints.sol

Since in this contract the structure (-s) is repeatedly written to the memory. Also, I suspect that such structure reading can create an overhead (since self is read multiple times).

However, this may require changing relatively many lines of code, and I'm not sure that such changes are desired for version 5.0. So I would like to hear the opinion of the maintainers on this. I'm not sure if it's worth creating a separate issue for this, but if necessary, I can create one.

Open to this experiment if you want to give it a shot, but agree with doing it for 5.1 since we're now focused on quality control for 5.0. Feel free to open a PR though we may just keep it open until post-5.0.