json_view: view_mode enum instead of bool Editable, node_kind for node::kind

basic_json_document and basic_json_view take a view_mode (read_only, the
default, or editable) instead of a bool: basic_json_document<json, true>
did not say what true means. The shortcut aliases are unchanged.

The kind of a node is a node_kind (the value_t values plus link, 10 as
before), which removes the static_cast<std::uint8_t>(value_t::...) of
every comparison. The node and image layouts are unchanged.

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

include/nlohmann/detail/view/serializer.hpp

@@ -874,7 +874,7 @@ class view_serializer
                 case value_t::object:
                 case value_t::array:
                 {
-                    const bool object = n->kind == static_cast<std::uint8_t>(value_t::object);
+                    const bool object = n->kind == node_kind::object;
                     room(8);
                     if (n->len == 0)
                     {