Add a vtkm::Vec<T,N> fast path for CopyInto a vec of the same type

Rather than use the general case which has a for loop we can just
use the assignment operator.

vtkm/Types.h

@@ -798,6 +798,8 @@ public:
     : Superclass(src)
   {
   }
+
+  inline VTKM_EXEC_CONT void CopyInto(Vec<T, Size>& dest) const { dest = *this; }
 };
 
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