FIX: Add fast case for dyn -> dyn conversion to speed up .into_dyn()

This method was simply not special cased for `IxDyn`.

Author: bluss

src/dimension/dimension_trait.rs

@@ -929,6 +929,11 @@ impl Dimension for IxDyn {
     fn from_dimension<D2: Dimension>(d: &D2) -> Option<Self> {
         Some(IxDyn(d.slice()))
     }
+
+    fn into_dyn(self) -> IxDyn {
+        self
+    }
+
     private_impl! {}
 }