* lisp/calendar/calendar.el: Use lexical-binding

* lisp/calendar/calendar.el: Use lexical-binding

(calendar-generate-window): Remove unused variable `day'.
(calendar-generate-month): Use calendar-dlet* to provide the dynbind
vars promised by the respective docstrings.
(calendar-update-mode-line): Use calendar-dlet* to provide `date' to
calendar-mode-line-format.  Don't call `eval' here since it's called in
calendar-string-spread anyway!
(calendar-date-string): Use calendar-dlet* to provide the dynbind
vars promised by the docstring of calendar-date-display-form.

* lisp/calendar/diary-lib.el (diary--date-string): Rename from date-string.