address inflation of model_stack object size after saving and reloading

[simonpcouch]

Closes #214.

Model stacks will still be larger after saving/reloading if not butchered, but when butchered, the saved and reloaded object will be comparable in size to the original. With this PR:

# object after reloading
lobstr::obj_size(stack_finalized)
#> 2.62 GB
# original object
lobstr::obj_size(stack_finalized_orig)
#> 1.63 GB

# butchered object after reloading
lobstr::obj_size(stack_finalized_butchered)
#> 48.59 MB
# butchered original object
lobstr::obj_size(stack_finalized_butchered_orig)
#> 47.14 MB