Add free functions for using apply directly on vectors.

[akleeman]

This change lets you do things like:

std::vector<T> foo;
const auto bar = apply(foo, convert_foo_to_bar);

It's not unlike std::transform but supports situations where the apply function returns void and doesn't require knowing the type of the transformed type ahead of time.

[jbangelo]

Looks good. It seems like they are essentially doing std::for_each or std::transform?

[akleeman]

@jbangelo yeah ... they pretty much are. I just dislike the syntax for transform etc. (Though maybe I should use it internally).

[jbangelo]

I don't see a reason to suddenly start. I agree that they are clunky to use, requiring the pairs of iterators. The "range" versions in C++20 are nicer to use.