* lisp/shadowfile.el: Use lexical-binding

* lisp/shadowfile.el: Use lexical-binding

Delete redundant `:group` args.

(shadow-hashtable): Make it an actual hash-table.
(shadow-shadows-of, shadow-invalidate-hashtable): Adjust accordingly.
(shadow-insert-var): Strength-reduce `eval` to `symbol-value`.
(shadow--save-buffers-kill-emacs): New function extracted from
`shadow-save-buffers-kill-emacs`.
(shadow-save-buffers-kill-emacs): Use it and use `save-buffers-kill-emacs`.
(shadow-initialize, shadowfile-unload-function):
Use `advice-add/remove` rather than override `save-buffers-kill-emacs`
with `defalias`.