This uses a macro, which can be a bit more brittle than an alias
template. However, class template argument deduction for alias templates
is only implemented in clang-19.
* The comment is wrong claiming that void* was returned when void was
returned in reality.
* The namespace is missing a name, leading to compile errors that are
suppressed with non-standard pragmas, and leading to compile errors in
future commits. Instead of using more non-standard suppressions, just
add the missing name.
* The SpanableYes/No types are missing begin/end iterators, which will
be needed when using std::span.
Also add test to make sure this doesn't get broken in the future.
This was breaking vector<bool> serialization in multiprocess code because
template current deduction guides would make it appear like vector<bool> could
be converted to a span, but then the actual conversion to span would fail.
