perf(vm): Reduce suspended Vm memory footprint

[aapoalas]

I ended up doing this absolutely on the side: I started thinking if keeping the entire Vm on the heap made sense. Avoiding copying the result, exception and reference values is an unquestionable win: These should not have any values that would be required after resuming since these are effectively "scratch space". Or at least I think that's the case :)

Turning the Vecs into Box<[T]> is is much less clear. If the Vec has zero capacity, then this is an unquestionable win as we just remove one cap: 0usize from the heap. If the Vec is empty but has non-zero capacity then this is a maybe-win: This is a dealloc of the vector's backing heap array. If the Vec is non-empty (and is not full) then this is probably a loss, as it means we dealloc the old backing heap array and realloc a new, smaller array. This is kind of unlikely to really win us any prizes.

The real question then becomes: How likely are these things?