Add Packing library (#4992)

Co-authored-by: ernestognw <ernestognw@gmail.com>

test/utils/Packing.t.sol

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.20;
+
+import {Test} from "forge-std/Test.sol";
+import {Packing} from "@openzeppelin/contracts/utils/Packing.sol";
+
+contract PackingTest is Test {
+    using Packing for *;
+
+    // Pack a pair of arbitrary uint128, and check that split recovers the correct values
+    function testUint128x2(uint128 first, uint128 second) external {
+        Packing.Uint128x2 packed = Packing.pack(first, second);
+        assertEq(packed.first(), first);
+        assertEq(packed.second(), second);
+
+        (uint128 recoveredFirst, uint128 recoveredSecond) = packed.split();
+        assertEq(recoveredFirst, first);
+        assertEq(recoveredSecond, second);
+    }
+
+    // split an arbitrary bytes32 into a pair of uint128, and check that repack matches the input
+    function testUint128x2(bytes32 input) external {
+        (uint128 first, uint128 second) = input.asUint128x2().split();
+        assertEq(Packing.pack(first, second).asBytes32(), input);
+    }
+}