allow PStrings in list construction directly

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/builtins/ListNodes.java

@@ -53,6 +53,7 @@
 import com.oracle.graal.python.builtins.objects.ints.PInt;
 import com.oracle.graal.python.builtins.objects.list.PList;
 import com.oracle.graal.python.builtins.objects.slice.PSlice;
+import com.oracle.graal.python.builtins.objects.str.PString;
 import com.oracle.graal.python.builtins.objects.tuple.PTuple;
 import com.oracle.graal.python.builtins.objects.type.PythonClass;
 import com.oracle.graal.python.nodes.PBaseNode;
@@ -303,6 +304,11 @@ public final PList execute(Object value, PythonClass valueClass) {
 
         public abstract PList execute(Object cls, Object value, PythonClass valueClass);
 
+        @Specialization
+        public PList listString(PythonClass cls, PString arg, PythonClass valueClass) {
+            return listString(cls, arg.getValue(), valueClass);
+        }
+
         @Specialization
         public PList listString(PythonClass cls, String arg, @SuppressWarnings("unused") PythonClass valueClass) {
             char[] chars = arg.toCharArray();