string: use static operator() for simple transform equality functors

string.hpp

@@ -401,10 +401,10 @@ namespace cruft::string::equality {
     /// \tparam TransformV   A character transform function
     template <char (*TransformV)(char) noexcept>
     struct transform {
-        bool operator() (
+        static bool operator() (
             std::string_view const &a,
             std::string_view const &b
-        ) const noexcept {
+        ) noexcept {
             if (a.size () != b.size ())
                 return false;