Fill method
Writes each sub-array in arrays into a tracked slot via rawArrayFunc
and returns the outer Pointer<Pointer<T>>]
Implementation
Pointer<Pointer<T>> Fill(List<List<X>> arrays, {String? key}) {
final pp = At(_slotKey(key), arrays.length);
for (int i = 0; i < arrays.length; i++) pp[i] = rawArrayFunc(arrays[i]);
return pp;
}