## PoolId Library
在 PoolId.sol 合约中定义了 `PoolId` 类型，其实际上就是 `bytes32`：
```solidity
import {PoolKey} from "./PoolKey.sol";
type PoolId is bytes32;
/// @notice Library for computing the ID of a pool
library PoolIdLibrary {
/// @notice Returns value equ...