util/TemplateString: add StringView cast operator

src/util/TemplateString.hxx

@@ -30,6 +30,8 @@
 #ifndef TEMPLATE_STRING_HXX
 #define TEMPLATE_STRING_HXX
 
+#include "StringView.hxx"
+
 #include <array> // for std::size()
 #include <cstddef>
 #include <string_view>
@@ -48,6 +50,10 @@ struct Buffer {
 	constexpr operator std::string_view() const noexcept {
 		return {value, size};
 	}
+
+	constexpr operator StringView() const noexcept {
+		return {value, size};
+	}
 };
 
 /**