FoR encoding should convert to unsinged variant of the original type

[robert3005]

In Vortex we have an issue where FoR arrays might no longer be sorted after compression spiraldb/vortex#400. In vortex it makes sense to try to find a different compressor in cases where FoR encoding would overflow. We could consider doing the same here since those values will not bitpack well

[robert3005]

Everything can continue to behave as expected if the array after for encoding is converted to unsigned variant of the original type.