* srfi-13.c (s_scm_string_map): convert character to unsigned char

* srfi-13.c (s_scm_string_map): convert character to unsigned char
before converting to unsigned int. This prevents hi-bit ascii as
being converted large integers.
(string_upcase_x): change caller for scm_{up,down}case to
scm_c_{up,down}case

* chars.h (scm_init_chars): change scm_{upcase,downcase} to
scm_c_{up,down}case.
(SCM_MAKE_CHAR): add (unsigned char) cast. This prevents havoc
when hi-bit ASCII is subjected to SCM_MAKE_CHAR().