Enforce mutability declarations in classes; correct shapes for classes

1. Enforce mutability declarations on class fields. Don't allow any
mutation of class fields not declared as mutable (except inside the
constructor).

2. Handle classes correctly in shape (treat classes like records).

src/test/compile-fail/mutable-class-fields.rs

@@ -0,0 +1,15 @@
+// error-pattern:assigning to immutable field
+class cat {
+  priv {
+    let mutable meows : uint;
+  }
+
+  let how_hungry : int;
+
+  new(in_x : uint, in_y : int) { meows = in_x; how_hungry = in_y; }
+}
+
+fn main() {
+  let nyan : cat = cat(52u, 99);
+  nyan.how_hungry = 0;
+}