Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/StorageSlotMock.sol

@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../utils/StorageSlot.sol";
+
+contract StorageSlotMock {
+    using StorageSlot for *;
+
+    function setBoolean(bytes32 slot, bool value) public {
+        slot.getBooleanSlot().value = value;
+    }
+
+    function setAddress(bytes32 slot, address value) public {
+        slot.getAddressSlot().value = value;
+    }
+
+    function setBytes32(bytes32 slot, bytes32 value) public {
+        slot.getBytes32Slot().value = value;
+    }
+
+    function setUint256(bytes32 slot, uint256 value) public {
+        slot.getUint256Slot().value = value;
+    }
+
+    function getBoolean(bytes32 slot) public view returns (bool) {
+        return slot.getBooleanSlot().value;
+    }
+
+    function getAddress(bytes32 slot) public view returns (address) {
+        return slot.getAddressSlot().value;
+    }
+
+    function getBytes32(bytes32 slot) public view returns (bytes32) {
+        return slot.getBytes32Slot().value;
+    }
+
+    function getUint256(bytes32 slot) public view returns (uint256) {
+        return slot.getUint256Slot().value;
+    }
+
+    mapping(uint256 => string) public stringMap;
+
+    function setString(bytes32 slot, string calldata value) public {
+        slot.getStringSlot().value = value;
+    }
+
+    function setStringStorage(uint256 key, string calldata value) public {
+        stringMap[key].getStringSlot().value = value;
+    }
+
+    function getString(bytes32 slot) public view returns (string memory) {
+        return slot.getStringSlot().value;
+    }
+
+    function getStringStorage(uint256 key) public view returns (string memory) {
+        return stringMap[key].getStringSlot().value;
+    }
+
+    mapping(uint256 => bytes) public bytesMap;
+
+    function setBytes(bytes32 slot, bytes calldata value) public {
+        slot.getBytesSlot().value = value;
+    }
+
+    function setBytesStorage(uint256 key, bytes calldata value) public {
+        bytesMap[key].getBytesSlot().value = value;
+    }
+
+    function getBytes(bytes32 slot) public view returns (bytes memory) {
+        return slot.getBytesSlot().value;
+    }
+
+    function getBytesStorage(uint256 key) public view returns (bytes memory) {
+        return bytesMap[key].getBytesSlot().value;
+    }
+}