strong: add three way comparison

Author: felixguendling

include/cista/strong.h

@@ -116,6 +116,10 @@ struct strong {
     return *this;
   }
 
+#if __cplusplus >= 201907L
+  constexpr auto operator<=>(strong const&) const = default;
+#endif
+
   constexpr bool operator==(strong const& o) const { return v_ == o.v_; }
   constexpr bool operator!=(strong const& o) const { return v_ != o.v_; }
   constexpr bool operator<=(strong const& o) const { return v_ <= o.v_; }