Fill method
Fills a tracked Pointer<Pointer<Char>> of count elements by calling
init for each index.
Implementation
Pointer<Pointer<Char>> Fill(int count, Pointer<Char> Function(int) init, {String? key}) {
final pp = At(_slotKey(key), count);
for (int i = 0; i < count; i++) pp[i] = init(i);
return pp;
}