FillWith method
Fills a tracked slot of count structs by setting each element to the
C returned by init(ptr, i).
Implementation
P FillWith(int count, V Function(P, int) init, {String? key}) {
final p = At(slotKey(key), count);
for (int i = 0; i < count; i++) indexSetterFunc(p, i, init(p, i));
return p;
}