the maximum length of str to use. If len < 0, then
the string is nul-terminated.
size_t * items_written
location to store the number of characters in the
result, or NULL.
DESCRIPTION
Convert a string from UTF-8 to a 32-bit fixed width
representation as UCS-4, assuming valid UTF-8 input.
This function does no error checking on the input.
RETURN VALUE
a pointer to a newly allocated UCS-4 string.
This value must be freed with free().
