S9 LIB  (string-expand string)          ==>  string
        (string-expand string integer)  ==>  string

Expand horizontal tabulation (HT, ASCII 9, aka "TAB") characters
in STRING to spaces. Return a new string. When INTEGER is specified,
expand each TAB to INTEGER spaces at maximum (default = 8).

(let ((tab (integer->char 9)))
  (string-expand (string #\x tab #\y)))  ==>  "x       y"
