Make Storage class completely stateless

The old style ArrayHandle stored most of its state, including the data, in the vtkm::cont::internal::Storage object (templated to the type of array). The new style of ArrayHandle stores the data itself in Buffer objects, and recent changes to Buffer allow metadata to be stored there, too.

These changes make it pretty unnecessary to hold any state at all in the Storage object. This is good since the sharing of state from one type of ArrayHandle to another (such as by transforming the data), can be done by just sharing the Buffer objects.

To reinforce this behavior, the Storage object has been changed to make it completely stateless. All the methods of Storage must be marked as static.