Make iterator_proxy_value a forward_iterator (#4371) (#4372)

* Make iteration_proxy_value model forward_iterator

* Amalgamate

* Suppress tidy warning

tests/src/unit-readme.cpp

@@ -171,7 +171,7 @@ TEST_CASE("README" * doctest::skip())
 
             // find an entry
             CHECK(o.find("foo") != o.end());
-            if (o.find("foo") != o.end())
+            if (o.find("foo") != o.end()) // NOLINT(readability-container-contains)
             {
                 // there is an entry with key "foo"
             }