title: Why does std::span lack size_type?
body: <p>I've been updating old code that used my homebrew span class to the one that is more in line with C++20 <code>std::span</code> and I'm getting compile errors because <code>std::span</code> doesn't have <code>size_type</code> and instead has <code>index_type</code>. There's been a huge drama over whether <code>index_type</code> should be signed or not, but why skipping <code>size_type</code>? This breaks generic code that expects containers (or container-like objects) to have <code>size_type</code>.</p>