[svn r117] Initial working implementation of interfaces.

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

test/interface4.d

@@ -0,0 +1,33 @@
+module interface4;
+
+interface I
+{
+    void func();
+}
+
+interface I2
+{
+    void func();
+}
+
+class C : I,I2
+{
+    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();
+    I2 i2 = c;
+    i2.func();
+    printf("final %d\n", c.i);
+    assert(c.i == 45);
+}