Add procedure string? defined in 6.3.5  Strings.

procedure:  (string? obj)

http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-2.html#%_toc_%_sec_6.3.5
Add procedure make-string defined in 6.3.5  Strings.

procedure:  (make-string k)

http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-2.html#%_toc_%_sec_6.3.5
Support characters defined in 6.3.4  Characters.

http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-2.html#%_toc_%_sec_6.3.4
Add procedures defined in 6.3.4  Characters.

procedure:  (char? obj)
procedure:  (char=? char1 char2)
procedure:  (char<? char1 char2)
procedure:  (char>? char1 char2)
procedure:  (char<=? char1 char2)
procedure:  (char>=? char1 char2)

http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-2.html#%_toc_%_sec_6.3.4
Add more procedures defined in 6.3.4  Characters.

procedure:  (char->integer char)
procedure:  (integer->char n)

http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-2.html#%_toc_%_sec_6.3.4