Save big amount of gas by using uint256 instead of bool in mapping

Booleans are more expensive than uint256 or any type that takes up a full word because each write operation emits an extra SLOAD to first read the slot's contents, replace the bits taken up by the boolean, and then write back.

mapping(address => mapping(address => bool)) private _operatorApprovals;
mapping(address => bool) public sharedProxyAddresses;

By just using mapping(address => uint256), and having:

• uint256 NOT_APPROVED = 1 instead of false
• uint256 APPROVED = 2 instead of true

there can be a considerable difference on gas saved.

Recommended read: Save over a hundred thousand gas with this Solidity gas optimization tip.

Replace boolean mappings with uint256 sentinels using 1 and 2 as the off and on states respectively to avoid the SLOAD on every write.