Enable `char` to be negative

From gtypes.h:

typedef char   gchar;

This means gchar can be signed as C char. Also, when we get a char from non-ascii strings, we can also get nagative value:

void main () {
    string s = "😆";
    for (var i = 0; i < s.length; i += 1) {
        print ("%d ", s.str[i]);
    }
}