Translate self referencing initialiser

[lucic71]

Fixes the original issue reported in #248. Self-refrencing in constructors is not handled here.

It translates:

struct list_head list = { &list };

to:

   let list: Value<list_head> = <Value<list_head>>::default();
    *list.borrow_mut() = list_head {
        next: (list.as_pointer()),
    };

That is, first create a default value, then assign it the self-referencing initialiser. Types that have defaults with side-effects are not supported.

[nunoplopes]

This is not a good nor generic solution. We should try to use Rc::new_cyclic.

[lucic71]

This is not a good nor generic solution. We should try to use Rc::new_cyclic.

I already thought about Rc::new_cyclic and it's not a good solution. It does not work for list_head called = {init(&called)};, i.e. initialiser writes the self-referenced variable.

Inside the Rc::new_cycle closure we only have a Weak without a constructed T. This mean that we can convert called to Ptr, but we cannot do read/write on the Ptr because Weak::upgrade will panic.

I think the generic solution (for self-referencing variables and for self-referencing constructors, see #248) is:

1. Initialize the variable in an empty-but-valid state, for example all fields zeroed. Default is not a good empty-but-valid state because it might have side effects
2. For self-referencing variables, write the initialiser in the storage from 1, basically the solution in this PR. For self-referencing constructors, pass the storage from 1 to the constructor call: T::new(&storage, arg1, arg2, ...)