Add a container type that is not coupled to a graph

[aucampia]

Currently, RDFLib has a container for lists that is coupled to a graph, but this makes it less versatile. If, for example, we want to write tests for lists, the graph has to be defined at the same time as the list, so writing parameterized tests is quite challenging.

Having a container type that is entirely decoupled from a graph would make it easier to write tests that involve lists.

[aucampia]

This PR has some test code which could be much simpler if RDFLib provided better ways for dealing with lists:

• https://github.com/RDFLib/rdflib/pull/2517