removeAt method
Removes the entry at visible position index, shifting the suffix
left by one.
Implementation
void removeAt(int index) {
final removed = _orderNids[index];
// memmove: shift [index + 1, _len) to [index, _len - 1).
_orderNids.setRange(index, _len - 1, _orderNids, index + 1);
_len--;
_onNidRemoved?.call(removed);
_onMutated();
}