[svn r117] Initial working implementation of interfaces.

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

test/interface2.d

@@ -0,0 +1,35 @@
+module interface2;
+
+interface A
+{
+    void a();
+}
+
+interface B
+{
+    void b();
+}
+
+class C : A,B
+{
+    int i = 0;
+    override void a()
+    {
+        printf("hello from C.a\n");
+    }
+    override void b()
+    {
+        printf("hello from C.b\n");
+    }
+}
+
+void main()
+{
+    scope c = new C;
+    {c.a();
+    c.b();}
+    {A a = c;
+    a.a();}
+    {B b = c;
+    b.b();}
+}