Add std::set<> typemaps for C#

Create new Lib/csharp/std_set.i based on the existing std_map.i and run
li_std_set unit test for C# as well.

Notice that the set operations defined by the base ISet<> interface are
not implemented yet.

Examples/test-suite/csharp/li_std_set_runme.cs

@@ -0,0 +1,55 @@
+using System;
+using System.Collections.Generic;
+using li_std_setNamespace;
+
+public class runme
+{
+    static void checkThat(bool mustBeTrue, string message)
+    {
+        if (!mustBeTrue)
+            throw new Exception("Test that the set " + message + " failed");
+    }
+
+    static void Main()
+    {
+        StringSet ss = new StringSet();
+
+        // Check the interface methods first.
+        ISet<string> s = ss;
+
+        checkThat(s.Count == 0, "is initially empty");
+        checkThat(!s.Contains("key"), "doesn't contain inexistent element");
+        checkThat(!s.Remove("key"), "returns false when removing inexistent element");
+
+        checkThat(s.Add("key"), "returns true when adding a new element");
+        checkThat(!s.Add("key"), "returns false when adding an existing element");
+        checkThat(s.Contains("key"), "contains the just added element");
+        checkThat(s.Remove("key"), "returns true when removing an existing element");
+        checkThat(s.Count == 0, "is empty again");
+
+        checkThat(s.Add("key1"), "Add(key1) returns true");
+        checkThat(s.Add("key2"), "Add(key2) returns true");
+        checkThat(s.Add("key3"), "Add(key3) returns true");
+
+        // Also check a different interface, providing a different Add() (sic!).
+        ICollection<string> coll = ss;
+        coll.Add("key");
+        checkThat(ss.Count == 4, "contains 4 elements");
+
+        // Now use object-specific methods, mimicking HashSet<>.
+        string val;
+        checkThat(ss.TryGetValue("key1", out val), "could retrieve existing item");
+        checkThat(val.Equals("key1"), "value was returned correctly by TryGetValue()");
+        checkThat(!ss.TryGetValue("no-such-key", out val), "couldn't retrieve inexistent item");
+        checkThat(val == null, "value was reset after failed TryGetValue()");
+
+        IList<string> list = new List<string>();
+        foreach (string str in ss) {
+            list.Add(str);
+        }
+        checkThat(list.Count == 4, "copy contains 4 elements");
+
+        ss.Clear();
+        checkThat(ss.Count == 0, "is empty after Clear()");
+    }
+}