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