boost::span cannot be implicitly constructed from `std::initializer_list` indirectly [feature-request]

[hadrielk]

NOTE: This isn't something std::span in C++20 supports either, unfortunately. But since Boost tries to offer things a future standard might adopt, I think this fits?

Because boost::span has no constructor signature with a direct std::initializer_list argument, but only a templated "range" one, this works:

std::initializer_list<int> il{1,2,3,4};
boost::span<int> sp(il);

but this does not, because it requires two implicit conversions to occur:

boost::span<int> sp({1,2,3,4});

I believe the above would work if this were to be added:

    constexpr span(std::initializer_list<T> il) noexcept
        : s_(boost::data(il), il.size())
    {}

I can provide a PR, if you wish.

I recognize this can lead to lifetime issues for the backing array, but that's already true for span regardless.

There is a std proposal for this, I believe: p2447r2, but I don't know its disposition.