Fill method
Fills a tracked slot of count structs, producing each element via
init(i) and writing it through writeIntoIndexedFunc.
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++) writeIntoIndexedFunc(p, i, init(i));
return p;
}