json_view: no save() and load() on big-endian targets

The image format is little-endian only. Instead of throwing
type_error.320 at runtime, save_image(), load_image() and the members
save() and load() do not exist on a big-endian target, so a call fails to
compile, like set() on a read-only document. type_error.320 remains for
saving a discarded document.

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

single_include/nlohmann/json.hpp

@@ -5565,7 +5565,7 @@ enum class exception_id : int
     type_not_serializable = 317, ///< a value cannot be serialized to the requested binary format
     enum_key_duplicate = 318, ///< an enum-keyed map has two keys that serialize to the same string
     document_binary_value = 319, ///< a binary value is stored in a json_document
-    document_image_unsupported = 320, ///< a json_document image cannot be saved or loaded (discarded document, big-endian target)
+    document_image_unsupported = 320, ///< a discarded json_document cannot be saved as an image
     discarded_value_used = 321, ///< a discarded value is used
     // out_of_range
     array_index_out_of_range = 401, ///< an array index is out of range