Remove Value from struct fields

[lucic71]

struct Item {
  int id;
  char name[8];
  std::vector<int> refs;
};

currently becomes in the refcount model:

pub struct Item {
    pub id: Value<i32>,
    pub name: Value<Box<[u8]>>,
    pub refs: Value<Vec<i32>>,
}

This breaks 2 patterns:

• field writes of reinterpreted structs are lost. *reinterpreted_p.upgrade().deref().field.borrow_mut = 42 loses the field write because reinterpreted_p.upgrade().deref() is a temporary
• taking the address of a field from a reinterpreted struct is dangling. reinterpreted_p.upgrade().deref().field.as_pointer() dangles because reinterpreted_p.upgrade().deref() is a temporary

The solution is to write the struct as:

pub struct Item {
    pub id: i32,
    pub name: Box<[u8]>,
    pub refs: Vec<i32>,
}

And add a new PtrKind for field pointers that saves a pointer to the parent struct + an offset into the struct.