Added type to variable declaration

The variable topSort was introduced as `topSort = []` without declared
type. It is assigned in the body of the local function strongConnect.
However, we only do local type inference, not across function
boundaries.

As  a result, at the return of stronglyConnectedComponentsAndTopSort,
the inferred type of topSort is still list[void] and the returned tuple
is rejected by the type checker.

src/org/rascalmpl/library/analysis/graphs/Graph.rsc

@@ -67,8 +67,8 @@ tuple[set[set[&T]], list[&T]]  stronglyConnectedComponentsAndTopSort(Graph[&T] g
     set[&T] onStack = {};       // set of nodes on current stack
     list[&T] stack = [];        // node stack contains nodes of SCC under construction
     
-    components = {};            // set of SCCs to be constructed
-    topsort = [];
+    set[set[&T]]components = {};// set of SCCs to be constructed
+    list[&T] topsort = [];      // sorted list of elements
     
     void strongConnect(&T v){
         // Set the depth index for v to the smallest unused index