`@unmanaged` is most likely leaking

[tomusdrw]

I think I had a wrong understanding of what @unmanaged does, so it seems that the code will now be leaking heap memory.

Would be best to figure out how to return multiple values in a way that doesn't require heap allocation.

[tomusdrw]

Most heap allocations removed in #88 ideally we would target --runtime=stub, but I guess that would complicate passing-in input programs (the host would need to handle allocation).