Use ArrayAccessor and FromIterator in Cast Kernels

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

The cast kernels are currently implemented using macros, and special cased for the various types of array. It would be less code, and easier to maintain if these instead used generics

Describe the solution you'd like

Rework the kernel to instead use a combination of FromIterator and ArrayAccessor instead of macros

Describe alternatives you've considered

We could not do this

Additional context

#2135