Expose the Variant helper class

For several versions, VTK-m has had a Variant templated class. This acts like a templated union where the object will store one of a list of types specified as the template arguments. (There are actually 2 versions for the control and execution environments, respectively.)

Because this is a complex class that required several iterations to work through performance and compiler issues, Variant was placed in the internal namespace to avoid complications with backward compatibility. However, the class has been stable for a while, so let us expose this helpful tool for wider use.

Fixes #735 (closed)