[svn r117] Initial working implementation of interfaces.

Groundwork for all the different types of class/interface casts laid out.

test/interface3.d

@@ -0,0 +1,28 @@
+module interface3;
+
+interface I
+{
+    void func();
+}
+
+class C : I
+{
+    int i = 42;
+    override void func()
+    {
+        printf("hello %d\n", i);
+        i++;
+    }
+}
+
+void main()
+{
+    scope c = new C;
+    {c.func();}
+    {
+        I i = c;
+        {i.func();}
+    }
+    {printf("final %d\n", c.i);}
+    {assert(c.i == 44);}
+}