boost::basic_string_view should have a trivial constructor from std::basic_string_view

I am trying to use beast which makes heavy use of boost::string_view in code that already makes heavy use of std::string_view. It is awfully easy to make a constexp constructor that trivially converts a std::string_view to a boost::string_view. I added one myself but I would appreciate it if you guys would go ahead and add it in too.

```
BOOST_CONSTEXPR basic_string_view(const std::basic_string_view& str) BOOST_NOEXCEPT
: ptr_(str.data()), len_(str.size()) {}
```