R4RS 6.6  (char->integer char)     ==>  integer
          (integer->char integer)  ==>  char

Given a character, CHAR->INTEGER returns an integer representation
of the character. Given an integer that is the image of a character
under CHAR->INTEGER, INTEGER->CHAR returns that character. These
procedures implement injective order isomorphisms between the set
of characters under the CHAR<=? ordering and some subset of the
integers under the <= ordering. That is, if

     (char<=? a b)  ==>  #t
and  (<= x y)       ==>  #t

and X and Y are in the domain of INTEGER->CHAR, then

     (<= (char->integer a)
         (char->integer b))       ==>  #t
and  (char<=? (integer->char x)
              (integer->char y))  ==>  #t.
