Use lexical-binding in todo-mode.el

Use lexical-binding in todo-mode.el

Adjust code accordingly and make various minor improvements.

* lisp/calendar/todo-mode.el: Enable lexical-binding.
(dayname, monthname, day, month, year): Make forward defvars
of these keywords from macros defined in calendar.el; wrap
them in with-no-warnings.
(todo-files, todo-files-function, todo-date-pattern)
(todo-mode-line-function, todo-show, todo-forward-category)
(todo-edit-item--header, todo-set-category-number)
(todo-adjusted-category-label-length)
(todo-total-item-counts, todo-filter-items)
(todo-print-buffer-function, todo-convert-legacy-date-time)
(todo-category-number, todo-category-completions)
(todo-read-file-name, todo-read-category)
(todo-validate-name, todo-read-date)
(todo-set-show-current-file, todo-modes-set-1)
(todo-modes-set-2, todo-modes-set-3, todo-mode):
Use #' instead of ' to quote functions.
(todo-files): Use \' instead of $ in regexp.
(todo--files-type-list): New function.
(todo-default-todo-file, todo-category-completions-files)
(todo-filter-files, todo-multiple-filter-files)
(todo-reevaluate-default-file-defcustom)
(todo-reevaluate-category-completions-files-defcustom)
(todo-reevaluate-filter-files-defcustom): Use it.
(todo-show, todo-rename-file, todo-move-category)
(todo-edit-item--text, todo-edit-quit, todo-edit-item--header)
(todo-item-undone, todo-unarchive-items, todo-search)
(todo-filter-items, todo-filter-items-1, todo-find-item)
(todo-category-select, todo-read-date)
(todo-nondiary-marker-matcher, todo-date-string-matcher)
(todo-diary-expired-matcher, todo-convert-legacy-files)
(todo-read-category): Reformat to avoid code hiding behind a
more deeply embedded element.
(todo-forward-category, todo-set-category-number):
Use 'funcall' instead of 'apply'.
(todo-toggle-mark-item, todo-edit-item--diary-inclusion)
(todo-edit-category-diary-inclusion)
(todo-insert-sort-button, todo-insert-category-line)
(todo-multiple-filter-files): Mark unused local variables.
(todo-edit-item--header, todo-move-item, todo-print-buffer)
(todo-edit-item--header, todo-move-item, todo-check-file)
(todo-edit-item--next-key): Remove unused local variables.
(todo-insert-sort-button, todo-insert-category-line):
Use a closure instead of a backquoted lambda.
(todo-update-categories-display, todo-print-buffer): Simplify code.
(todo-print-buffer-function): Document calling convention.
(todo-category-completions): Use cl-pushnew instead of add-to-list.
(todo-mode-map, todo-archive-mode-map)
(todo-categories-mode-map, todo-filtered-items-mode-map):
Remove superfluous call of suppress-keymap, since it's already
in the parent special-mode-map.