Request support for boost::span

[Arsennnic]

I use boost::span as an alternative to std::span since I'm limited to C++17. boost::span has been introduced in Boost core since Boost-1.78, which is a compatible implementation of std::span.

Would you please and the support for boost::span?

Alternatively, would you mind if I refer to the code in <highfive/span.hpp>, implement this feature by myself, and open a pull request?