[string.view.synop][span.syn] Declaration for primary template and namespace of enable_safe_range

[frederick-vs-ja]

In N4842, the primary template of std::ranges::enable_safe_range is not declared in synopses of <string_view> and <span>, and current namespace of partial specializations might be wrong.

[CaseyCarter]

the primary template of std::ranges::enable_safe_range is not declared in synopses of <string_view> and <span>

We cannot define a template in multiple headers, and there are no non-defining declarations of variable templates in C++. I suppose we could put the primary template in an exposition-only header included by all of <span>, <string_view>, and <ranges>, but it seems like overkill to me. I'm perfectly happy to depict the specializations in individual headers and leave it to "implementor magic" to make that valid.

current namespace of partial specializations might be wrong

I believe I fixed this in #3551.