MemLeak persistent mode

[zachriggle]

The MemLeak module is really nifty, but it would be nice if there were a way to (optionally) automatically persist leaked memory to a file, such that subsequent script executions can load the cached / persisted leaked memory from file, instead of leaking again over-the-wire.

This would be useful for deterministic, memory-scanning-heavy operations like Pwnable.kr's Exynos challenge.