Added support for defer init for dyn_var

[AjayBrahmakshatriya]

Usually when dyn_var are instantiated, they are immediately initialized. This means they can be declared only inside builder contexts (or should use as_global or with_name).

This is restrictive when the user wants to create objects outside the context that have dyn_var members. Currently this requires the user to push the creation of the entire object inside the context. It creates issues with expensive computations being run multiple times. To get around this, we add a new constructor helper builder::defer_init and a member function deferred_init.

The constructor helper allows declaring dyn_var(s) without actually doing anything. The member function deferred_init, then does the job of the constructor and should be called inside the context.

Added sample53 to demonstrate/test this.