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

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:
```cpp
std::initializer_list il{1,2,3,4};
boost::span sp(il);
```

but this does not, because it requires two implicit conversions to occur:
```cpp
boost::span sp({1,2,3,4});
```

I believe the above would work if this were to be added:
```cpp
constexpr span(std::initializer_list 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](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2447r2.html), but I don't know its disposition.