Add compile-pass test about Self: Sized bounds on the trait

Currently, the `impl` block we generate for those traits is a bit
stupid as it contains `T: Trait + ?Sized` (where `Trait: Sized`), but
it compiles fine. So I don't see a reason to make our code more
complicated just to avoid this strangeness.

tests/compile-pass/trait_obj_self_sized.rs

@@ -0,0 +1,17 @@
+use auto_impl::auto_impl;
+
+
+#[auto_impl(Box)]
+trait Foo: Sized {
+    fn foo(&self);
+}
+
+#[auto_impl(Box)]
+trait Bar where Self: Sized {
+    fn foo(&self);
+}
+
+#[auto_impl(Box)]
+trait Baz: Sized where Self: Sized {
+    fn foo(&self);
+}