Add nd_type_p macro

[S-H-GAMELINKS]

MRI has these code using nd_type macro to check NODE type.

static NODE *
rest_arg_append(struct parser_params *p, NODE *args, NODE *rest_arg, const YYLTYPE *loc)
{
    NODE *n1;
    if ((nd_type(rest_arg) == NODE_LIST) && (n1 = splat_array(args)) != 0) {
	return list_concat(n1, rest_arg);
    }
    return arg_concat(p, args, rest_arg, loc);
}

However, conditions will be long and sometimes difficult to read, I think.

So, I think better to introduce nd_type_p macro like this.

#define nd_type_p(n, t) (nd_type((n)) == (t))

I think that conditions can be more shorter to using nd_type_p macro.
How about it？

static NODE *
rest_arg_append(struct parser_params *p, NODE *args, NODE *rest_arg, const YYLTYPE *loc)
{
    NODE *n1;
    if ((nd_type_p(rest_arg, NODE_LIST)) && (n1 = splat_array(args)) != 0) {
	return list_concat(n1, rest_arg);
    }
    return arg_concat(p, args, rest_arg, loc);
}

[nobu]

Another nd_type() == NODE_... is in a comment at compile.c:1784.

[S-H-GAMELINKS]

@nobu
Thank you for your review.
I fixed code that you review pointed out in this pull request.

[S-H-GAMELINKS]

Rebased to latest master(a84dc9d) branch.