[libc++] Avoid including vector in <functional>

[philnik777]

vector has been used in a very simple way in boyer_moore_searcher. We can instead just use unique_ptr<T[]>, which is a lot simpler, allowing us to drop the vector dependency while not losing any expressiveness in the code. As a nice side effect, this also reduces the time it takes to instantiate the boyer_moore_searcher constructor from 26ms to 22ms on my machine.