[RFC] Speed up the `test_it_generates_unique_ids()` by using `assertSame()` instead of `assertNotContains()`

[viccherubini]

Neat project! I love the simplicity and speed. I did some benchmarking and the test_it_generates_unique_ids() test uses assertNotContains() in a loop of 1000 iterations. On my machine, this test takes about 165312875 nanoseconds to run. When replacing the assertNotContains() call to a single assertSame() call at the end of the loop, the test drops to about 869500 nanoseconds, an improvement of 3 orders of magnitude.

public function test_it_generates_unique_ids()
{
    $ids = [];

    for ($i = 0; $i < 1000; $i++) {
        $ids[] = ObjectId::generate();
    }

    $this->assertSame($ids, array_unique($ids));
}

Would you like me to open a PR?