sylvan_bdd: rewritten internal referencing code
sylvan_bdd: rewritten internal referencing code

Now instead of INIT and EXIT we use explicit pop(...) to remove references from the stack.
This removes the need to remember the state of the internal reference stack.

utwente-fmt/sylvan,trolando/sylvan,trolando/sylvan,Meijuh/sylvan,Meijuh/sylvan,bernied/sylvan,utwente-fmt/sylvan,bernied/sylvan