Performance improvements for add-in-template fn

In order to improve performance and readability of the function
add-in-template in namespace io.pedestal.app.render.push.templates
i implemented following changes:

 - The function now walks over all fields in only one doseq loop
   instead of the 2 nested doseq loops as before, in order to better
   match the shape of the refactored update-template-fn in the same
   namespace

 - Because of the first point, we get rid of the repeated call to
   (get t k) in one loop pass and also unnecessary contains? checks