NFC: Add Concept for equality modulo references

src/ds/concept.h

@@ -39,6 +39,13 @@ namespace snmalloc
   concept ConceptSame = std::is_same<T, U>::value;
 #  endif
 
+  /**
+   * Equivalence mod std::remove_reference
+   */
+  template<typename T, typename U>
+  concept ConceptSameModRef =
+    ConceptSame<std::remove_reference_t<T>, std::remove_reference_t<U>>;
+
   /**
    * Some of the types in snmalloc are circular in their definition and use
    * templating as a lazy language to carefully tie knots and only pull on the