Optimize item spawn list creation

[BrettDong]

Summary

Performance "Optimize item spawn list creation"

Purpose of change

Around 7% of time in starting_items test case is just spent on concatenating std::vector<item>.

Current implementation of creating item spawn list is recursively doing

std::vector<item> create(...) {
    std::vector<item> result;
    std::vector<item> temp = create(...);
    result.insert( result.end(), temp.begin(), temp.end() );
    return result;
}

This results in a lot of temporary item and std::vector<item> construction, and std::vector<item> range copying.

Describe the solution

Instead of returning std::vector<item> container in the recursive algorithm, pass around the result std::vector<item> as a mutable reference in the parameters of the recursive algorithm, and construct items in place in the one std::vector<item> container.

Describe alternatives you've considered

Refactor the recursive algorithm to something iterative.

Testing

std::vector<item> data copy overhead is completely resolved. Reduces starting_items test time by 12 seconds on my computer. This optimization also benefits mapgen slightly.

Additional context

[akrieger]

Nice fix. I noticed this but didn't know enough about the item code to know how to approach this.

[BrettDong]

Your #55070 inspired me to look into inefficient use of temporary container concatenation.