emacs modules memory leak in 27

["Milan Stanojević" <mstanojevic@janestreet.com>]

This is on emacs-27 branch.

env-make_global_ref will add a reference to the underlying Lisp_Object
and allocate emacs_value from the global storage. env->free_global_ref
on the other hand will only remove a reference to the underlying
Lisp_Object and not free the emacs_value.

The global storage of emacs_values seems to be only growing and is
never collected (and I think with the current array implementation it
is probably not possible to collect).

This leak is not present in 26 and I think the leak was introduced
with the change in the underlying representation of emacs_value. (If
I'm reading git history right, since 26 this was changed and then
reverted and then changed again).