Currently, ListTags are implemented by having a subtype name list set to
a brigand::list. However, there is always a chance this will change. To
make things more explicit, create a vtkm::internal::ListTagToBrigandList
to make it clear what the resulting type should be (and provide some
potential future-proofing).

Also add a convenient vtkm::ListTagApply that allows you to  easily
instantiate a template with the list of types in a ListTag.