Support for rdf Lists

[hsolbrig]

Currently, one has to define a separate shape for every list type (List(Person), List(Class), List(Student) ...). Additional shape definitions have to be added to cover list cardinality (non-empty List of Person, List of two or more Students, etc). In addition, this puts the onus on the ShEx writer to correctly characterize a list -- something that is non-trivial (although a good way to learn ShEx details).

Would like to see that some sort of List idiom be added to the specification that allows one to specify type and cardinality, and that lets the hard work be done within the ShEx interpreter