S9 LIB  (string-scan char string)     ==>  integer | #f
        (string-ci-scan char string)  ==>  integer | #f

        (load-from-library "string-scan.scm")

Scan STRING for CHAR. When STRING contains CHAR, return its position,
otherwise return #F. STRING-CI-SCAN does the same but ignores case.

(string-scan #\y "xyz")  ==>  1
