Container access with non-member functions

Standard containers provide the begin() and end() member functions for retrieving iterators to the first and one-past-last element of the container. There are actually four sets of these functions. Apart from begin()/end(), containers provide cbegin()/cend() to return constant iterators, rbegin()/rend() to return mutable reverse iterators, and crbegin()/crend() to return constant reverse iterators. In C++11/C++14, all these have non-member equivalents that work with standard containers, C-like arrays, and any custom type that specializes them. In C++17, even more non-member functions have been added; std::data()--that returns a pointer to the block of memory containing the elements of the container, std::size()--that returns the size of a container or array, and std::empty()--that returns whether the given container is empty.  These non-member functions are intended...