Memleak documentation endianness

Update `pwnlib.memleak` to add doctests, as well as support `context.endianness`.  Currently, `memleak` is hard-coded to be little-endian.  Docstrings were fixed to remove constructs like `func(...) -> dword` in favor of `func(...) -> int`.  Also migrated the search functionality to the main loop in `pwnlib.memleak.Memleak._leak`.