Date#prev_year

static VALUE
d_lite_prev_year(int argc, VALUE *argv, VALUE self)
{
    VALUE n;

    rb_scan_args(argc, argv, "01", &n);
    if (argc < 1)
        n = INT2FIX(1);
    return d_lite_lshift(self, f_mul(n, INT2FIX(12)));
}