Provide general list and map classes

This came up during #1905230: Improve the typed data API usage of configuration schema and some other places, so let's do that.

We also need a generic item lists for handling multiple-valued contexts of #1743686: Condition Plugin System, or generic map to handle link field attributes.

Suggested names:
- list -> list. However the classname "List" is reserved, so I have used ItemList for now.
- complex data -> "map" ?

Attached is a first patch which starts extracting code from the Field class and moves it to an item list class.