Fixed bug with dyn_var<T*> requiring T to be completely defined

[AjayBrahmakshatriya]

BuildIt had a bug with how it handles dyn_var<T*> for recursive types. A very common pattern like the following appears in linked list implementations -

struct linked_list {
    dyn_var<linked_list*> next;
};

Typically declaring a pointer to a type doesn't require the type to be "complete". However the way dyn_var<T*> is implemented is it tries to create an object of type dyn_var inside to return when the ->/[] operator is called. Now this causes issues because dyn_var requires T to be "complete" since it inherits from T when T is a struct type.

This causes the above sample to fail with compile error.

To fix this we make the extra member of type dyn_var inside dyn_var<T*> be created lazily. We change the member to be of type std::unique_ptr<dyn_var> and allocate it when it is first required.

Sample63 has been added to test this