boost::size() and friends should use an ADL barrier like boost::begin

[Tavian Barnes <tavianator@…>]

boost::begin() and boost::end() are nicely declared in an ADL barrier namespace to avoid conflicts with code like this:

using std::begin;
auto i = begin(c);

​N4280 has been accepted into C++17, which adds std::size, std::empty, and std::data. But if users attempt to use those in the same way:

using std::size;
auto s = size(c);

They may get an error. For example, if c is a std::vector<boost::shared_ptr<int>> or something, boost::size() will be found by ADL, and since it's no more specific than std::size, the call will be ambiguous.