Implement stub for 'str.casefold'.

Author: fangerer

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/str/StringBuiltins.java

@@ -991,7 +991,7 @@ char[] doPString(char[] translatedChars, int i, PString translated) {
     @Builtin(name = "lower", minNumOfPositionalArgs = 1)
     @GenerateNodeFactory
     @TypeSystemReference(PythonArithmeticTypes.class)
-    public abstract static class LowerNode extends PythonBuiltinNode {
+    public abstract static class LowerNode extends PythonUnaryBuiltinNode {
 
         @Specialization
         @TruffleBoundary
@@ -2123,4 +2123,18 @@ PStringIterator doString(String self) {
         }
 
     }
+
+    @Builtin(name = "casefold", minNumOfPositionalArgs = 1)
+    @GenerateNodeFactory
+    @TypeSystemReference(PythonArithmeticTypes.class)
+    public abstract static class CasefoldNode extends PythonUnaryBuiltinNode {
+
+        @Specialization
+        String doString(String self) {
+            // TODO(fa) implement properly using 'unicodedata_db' (see 'unicodeobject.c' function
+            // 'unicode_casefold_impl')
+            return self.toLowerCase();
+        }
+
+    }
 }