The main point of this MR is to implement
CastAndCallWithExtractedArray in UnknownArrayHandle.
This provides a convenience for calling a function for most
ArrayHandle types.
This was used to update some functions like MapFieldMergeAverage
and MapFieldPermutation.
The extra use of RecombineVec defined by ArrayHandleRecombineVec
required several fixes to make the behavior of the values more
expected. For example, several assignment operators had to be
added or fixed with RecombineVec and other Vec objects.