Optimize storage for Result with NeverValueField about subspace HOT 1 OPEN

I forgot when writing this that Result also has a 3rd state: Ok, Err, and Moved (aka holding neither). And Result needs to track this and panic when used in Moved state to not give access to moved-from Ok or Err values. Option on move can be come empty, but Result becomes invalid.

The one case we could optimize storage still is for when T and E are both NeverValueField. Then we could store:

Option<T> ok_;
Option<E> err_;

and when both are empty, the Result is moved-from. And it would not need any additional state.

But we'd need to generalize NeverValueField to handle more than 1 possible value in order to ever optimize the case where only T or only E satisfy NeverValueField.

from subspace.