new tutorial example

doc/tutorial/StringOper.gf

@@ -0,0 +1,10 @@
+resource StringOper = {
+    oper
+      SS : Type = {s : Str} ;
+
+      ss : Str -> SS = \x -> {s = x} ;
+
+      cc : SS -> SS -> SS = \x,y -> ss (x.s ++ y.s) ;
+
+      prefix : Str -> SS -> SS = \p,x -> ss (p ++ x.s) ;
+}