Many of the operations of `VariantArrayHandleBase` are not dependent on
the TypeList parameter of the class. Still others can operate just as
well by providing a type list to a method. Thus, it is convenient to
create a superclass that is not templated. That allows us to pass around
a `VariantArrayHandle` when the type list does not matter.

This superclass is called `VariantArrayHandleCommon` because "base" was
already taken.