[poincare/expression_node] Fix ExpresssionNode::Type ordering

A Multiplication whose children are all real or ComplexCartesian are
multiplied to form a single ComplexCartesian. That step of shallowReduce
assumed that the last child is a ComplexCartesian. Since a Unit is now
considered as real, the previous assumption can be maintained be
reordering the ExpresssionNode::Type.

poincare/include/poincare/expression_node.h

@@ -95,6 +95,7 @@ public:
     Subtraction,
     Sum,
 
+    Unit,
     ComplexCartesian,
 
     ConfidenceInterval,
@@ -104,7 +105,6 @@ public:
     MatrixTranspose,
     PredictionInterval,
     Matrix,
-    Unit,
     EmptyExpression
    };