[svn r117] Initial working implementation of interfaces.

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

test/interface1.d

@@ -0,0 +1,22 @@
+module interface1;
+
+interface Inter
+{
+    void func();
+}
+
+class Class : Inter
+{
+    override void func()
+    {
+        printf("hello world\n");
+    }
+}
+
+void main()
+{
+    scope c = new Class;
+    c.func();
+    Inter i = c;
+    i.func();
+}