Reorder so that functions are defined before use.

Reorder so that functions are defined before use.
(displayed-month, displayed-year): Move declarations where needed.
(solar-get-number): Move definition before use.  Use unless.
(solar-equatorial-coordinates): Simplify.
(solar-sunrise-and-sunset): Use let rather than let*.
(solar-longitude, solar-equinoxes-solstices): Use cadr, nth