Rename `at()` functions

[wjmelements]

🧐 Motivation
Per https://docs.soliditylang.org/en/latest/contracts.html#custom-storage-layout, layout and at will become reserved words.

📝 Details
Several functions, such as EnumerableSet.at(uint256), use these keywords for their signature. This will not be allowed in a future version of Solidity, likely 0.9.

[ernestognw]

Hi @wjmelements, thanks for the heads up. It seems that we should update the at function probably for 6.x given it's a breaking change.

Is there any timeline known for Solidity 0.9? 🤔

[wjmelements]

I don't know the plan. It looks like 0.9 used to be tied to EOF but that's not happening anymore.
Most of the planned 0.9 items are incomplete but none of them look large so I would guess this is at least 6 months away.

Here are the 0.9 tasks.
https://github.com/ethereum/solidity/milestone/52

Here is the solidity keyword task.
argotorg/solidity#14770