Cache internal results

[alecandido]

This is something we're already doing in ekomark (it's part of the sandbox mode), and we can lift here.

In any case, it has a completely different role than external, and it should have even a different life-cycle: if we change the code and rerun, the internal cache should not be used, but nothing would have changed the external one.

So, we can do this with an extra table (e.g. InternalCache), and explicitly opt-in (even though I'd like to avoid interactive prompts, they are annoying in a lot of ways...).