[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();
+}

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();}
+}

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);}
+}

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);
+}