Make 'purecopy' an obsolete function alias for 'identity'

Make 'purecopy' an obsolete function alias for 'identity'

* lisp/subr.el (purecopy): New obsolete function alias for 'identity'.
* src/alloc.c (purecopy): Remove function.
(Fpurecopy): Remove DEFUN.
(syms_of_alloc): Remove defsubr for above DEFUN.
* lisp/loadup.el (purify-flag): Don't set to hash table.

* doc/lispref/spellfile:
* doc/lispref/keymaps.texi (Tool Bar):
* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns): Delete
references to 'purecopy'

(cherry picked from commit d6b05b128280cc23dc77a1a06194c4e69d1ac519)