R4RS 6.5.6  (string->number string)          ==>  integer
            (string->number string integer)  ==>  integer

Returns a number expressed by the given STRING. INTEGER must be a radix
of either 2, 8, 10, or 16. If no radix supplied, then the default radix
is 10. If STRING is not a syntactically valid notation for a number,
then STRING->NUMBER returns #F.

(string->number "100")     ==>  100
(string->number "100" 16)  ==>  256
