Add a Slots library, and a transient variant of ReentrancyGuard

.changeset/honest-planets-sell.md

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+---
+'openzeppelin-solidity': minor
+---
+
+`Slots`: Added a library to help derivate, read and write to storage slots (both "normal" and transient).

.changeset/hungry-oranges-travel.md

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+---
+'openzeppelin-solidity': minor
+---
+
+`TransientReentrancyGuard`: Added a variant of `ReentrancyGuard` that uses transient storage.

contracts/mocks/ReentrancyTransientMock.sol

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+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.24;
+
+import {ReentrancyGuardTransient} from "../utils/ReentrancyGuardTransient.sol";
+import {ReentrancyAttack} from "./ReentrancyAttack.sol";
+
+contract ReentrancyTransientMock is ReentrancyGuardTransient {
+    uint256 public counter;
+
+    constructor() {
+        counter = 0;
+    }
+
+    function callback() external nonReentrant {
+        _count();
+    }
+
+    function countLocalRecursive(uint256 n) public nonReentrant {
+        if (n > 0) {
+            _count();
+            countLocalRecursive(n - 1);
+        }
+    }
+
+    function countThisRecursive(uint256 n) public nonReentrant {
+        if (n > 0) {
+            _count();
+            (bool success, ) = address(this).call(abi.encodeCall(this.countThisRecursive, (n - 1)));
+            require(success, "ReentrancyTransientMock: failed call");
+        }
+    }
+
+    function countAndCall(ReentrancyAttack attacker) public nonReentrant {
+        _count();
+        attacker.callSender(abi.encodeCall(this.callback, ()));
+    }
+
+    function _count() private {
+        counter += 1;
+    }
+
+    function guardedCheckEntered() public nonReentrant {
+        require(_reentrancyGuardEntered());
+    }
+
+    function unguardedCheckNotEntered() public view {
+        require(!_reentrancyGuardEntered());
+    }
+}