Fill method
Allocates (or reuses) a slot of count elements, populating each index
i with the value returned by init(i).
Implementation
Pointer<T> Fill(int count, X Function(int) init, {String? key}) {
final p = At(_slotKey(key), count);
for (int i = 0; i < count; i++) indexSetterFunc(p, i, init(i));
return p;
}