Changes

Perft is mostly for debugging purposes. It works mainly with functions: move generators, make move, unmake move. They all are very basic and vital for chess engines. By comparing perft results developers can find out if those functions work correctly or not. If they are incorrect developers can narrow quickly which by comparing branches are wrong, then call Perft for those wrong branches with lower depth, repeat until finding direct positions gives which give the wrong result. Sometimes developers may compare Perft's speed to understand how good/bad their functions are.