[Cs3] upper_bound()/lower_bound() for unordered containers

[Mikhail Nesterenko]

CS3 students,

I mentioned I double check on their existence. It is official. They
are not implemented for unordered containers (hashes) since the
containers are unordered and these two functions do not make sense.

I updated the slides to reflect that.

Thanks,
-- 
Mikhail