Fixes #4029

This PR fixes typings of the parseLog function. Specifically, parseLog doesn't work with the Logs that you receive from various ethers methods (logs in transaction receipts, when doing an eth_getLogs, etc.).

The typing issue is fairly straight forward, the parseLog function expects topics to be a string[], but the Log.topics field is a readonly string[]. This causes some annoying TypeScript compile issues:

import { Interface, Log } from "ethers";

declare let log: Log;
declare let iface: Interface;
iface.parseLog(log)

error TS2345: Argument of type 'Log' is not assignable to parameter of type '{ topics: string[]; data: string; }'.
  Types of property 'topics' are incompatible.
    The type 'readonly string[]' is 'readonly' and cannot be assigned to the mutable type 'string[]'.

5 iface.parseLog(log)
                 ~~~

Fortunately, parseLog doesn't need topics to be mutable, so the fix is fairly straight-forward.