Address the review of json_view element access

From gregmarr's review of #5622:

- count() counts every member with the key, so count(key) > 1 detects
  duplicate keys; it was contains() as 0/1 before.
- get_to() passes types that get<T>() does not convert directly to
  from_json with the existing value, as basic_json::get_to() does, so that
  a from_json which sets only the fields present keeps the others.
- array_index() checks the empty token and "-" first; the results are
  unchanged.
- Docs: no "if an exception is thrown" where a view never changes
  anything; the borrowed source must also stay unmodified; items() on a
  temporary view is safe; back()/front() notes shortened; the basic_json
  see-also entries say how the view's results can differ.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/view/value.hpp

@@ -8,6 +8,7 @@
 
 #pragma once
 
+#include <cstddef> // nullptr_t, size_t
 #include <cstdint> // int64_t
 #include <map> // map
 #include <string> // basic_string
@@ -103,6 +104,27 @@ Map map_value(const View& v)
 template<typename Key>
 struct is_string_key : std::is_constructible<Key, const char*, std::size_t> {};
 
+/// whether View::get<T>() converts to T without materialize(); get_to()
+/// assigns these, and passes the others to from_json
+template<typename T, typename BasicJsonType, typename View>
+struct is_direct_value : std::integral_constant < bool,
+    std::is_arithmetic<T>::value || std::is_same<T, std::nullptr_t>::value
+    || std::is_same<T, typename View::string_view_t>::value
+    || std::is_same<T, BasicJsonType>::value || std::is_same<T, View>::value >
+{};
+
+template<typename Traits, typename Alloc, typename BasicJsonType, typename View>
+struct is_direct_value<std::basic_string<char, Traits, Alloc>, BasicJsonType, View> : std::true_type {};
+
+template<typename U, typename A, typename BasicJsonType, typename View>
+struct is_direct_value<std::vector<U, A>, BasicJsonType, View> : std::true_type {};
+
+template<typename K, typename V, typename C, typename A, typename BasicJsonType, typename View>
+struct is_direct_value<std::map<K, V, C, A>, BasicJsonType, View> : is_string_key<K> {};
+
+template<typename K, typename V, typename H, typename E, typename A, typename BasicJsonType, typename View>
+struct is_direct_value<std::unordered_map<K, V, H, E, A>, BasicJsonType, View> : is_string_key<K> {};
+
 }  // namespace view
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END