Add Packing library (#4992)

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

test/utils/Packing.t.sol

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+// 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);
+    }
+}

test/utils/Packing.test.js

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+const { ethers } = require('hardhat');
+const { expect } = require('chai');
+const { loadFixture } = require('@nomicfoundation/hardhat-network-helpers');
+const { generators } = require('../helpers/random');
+
+async function fixture() {
+  return { mock: await ethers.deployContract('$Packing') };
+}
+
+describe('Packing', function () {
+  beforeEach(async function () {
+    Object.assign(this, await loadFixture(fixture));
+  });
+
+  it('Uint128x2', async function () {
+    const first = generators.uint256() % 2n ** 128n;
+    const second = generators.uint256() % 2n ** 128n;
+    const packed = ethers.hexlify(ethers.toBeArray((first << 128n) | second));
+
+    expect(await this.mock.$asUint128x2(packed)).to.equal(packed);
+    expect(await this.mock.$asBytes32(packed)).to.equal(packed);
+    expect(await this.mock.$pack(first, second)).to.equal(packed);
+    expect(await this.mock.$split(packed)).to.deep.equal([first, second]);
+    expect(await this.mock.$first(packed)).to.equal(first);
+    expect(await this.mock.$second(packed)).to.equal(second);
+  });
+});