Get asprintf to work with string_view in the args list

Based on a question over in Zitrax's gist:
(https://gist.github.com/Zitrax/a2e0040d301bf4b8ef8101c0b1e3f1d5), we
use static_cast, combined with std::move and std::forward in
unistd::asprintf_convert to get unistd::asprintf to work with
std::string_view in the argument list.

There are two other variations on the return line that also work, but
I chose std::forward to keep things the same.

I also wonder if this might not introduce a memory leak?

include/unistd/asprintf.h

@@ -24,7 +24,11 @@ namespace unistd {
 
 template<typename T>
 auto asprintf_convert(T&& t) {
-  if constexpr (std::is_same_v<std::remove_cv_t<std::remove_reference_t<T>>, std::string>) {
+  if constexpr (std::is_same_v<std::remove_cv_t<std::remove_reference_t<T>>, std::string_view>) {
+    std::string s = std::move(static_cast<std::string>(t));
+    return std::forward<std::string>(s).c_str();
+  }
+  else if constexpr (std::is_same_v<std::remove_cv_t<std::remove_reference_t<T>>, std::string>) {
     return std::forward<T>(t).c_str();
   }
   else {