add ERC2771 interfaces and storage library

Author: ItsNickBarry

contracts/interfaces/IERC2771.sol

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.20;
+
+import { _IERC2771 } from './_IERC2771.sol';
+
+interface IERC2771 is _IERC2771 {
+    function isTrustedForwarder(address forwarder) external view returns (bool);
+}

contracts/interfaces/_IERC2771.sol

@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.20;
+
+interface _IERC2771 {}

contracts/storage/ERC2771Storage.sol

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+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.20;
+
+library ERC2771Storage {
+    /**
+     * @custom:storage-location erc7201:solidstate.contracts.storage.ERC2771
+     */
+    struct Layout {
+        mapping(address account => bool trustedStatus) trustedForwarders;
+    }
+
+    bytes32 internal constant DEFAULT_STORAGE_SLOT =
+        keccak256(
+            abi.encode(
+                uint256(
+                    keccak256(bytes('solidstate.contracts.storage.ERC2771'))
+                ) - 1
+            )
+        ) & ~bytes32(uint256(0xff));
+
+    function layout() internal pure returns (Layout storage $) {
+        $ = layout(DEFAULT_STORAGE_SLOT);
+    }
+
+    function layout(bytes32 slot) internal pure returns (Layout storage $) {
+        assembly {
+            $.slot := slot
+        }
+    }
+}