Update day6_2.rs mod andreselcientifico_david_a_perez

[andreselcientifico]

I made an improvement to David's code that made it more than 3 times faster.

[JonathanHallstrom]

have you verified that your code works?

            state |= 1 << bit_idx;
            state & (1 << bit_idx) != 0

correct me if im wrong but it seems like this expression would always evaluate to true?
(if im understanding correctly the compiler seems to agree https://godbolt.org/z/E67x4G85W)