if err != nil {
		return err
	}
	res, err := http.DefaultTransport.RoundTrip(req)
	if err != nil {
		return err
	}
	// Handle redirects.
	if res.StatusCode/100 == 3 {
		newURL, err := res.Location()
		if err != nil {
			return fmt.Errorf("resolving redirect: %v", err)
		}
		if !strings.HasPrefix(newURL.String(), *root) {
			// Skip off-site redirects.
			return nil
		}
		crawl(newURL.String(), url)

be inefficient for allocating only small amounts of small Go values. They're
best used in bulk, on the order of MiB of memory allocated on each use.

Note that by allowing for this limited form of manual memory allocation
that use-after-free bugs are possible with regular Go values. This package
limits the impact of these use-after-free bugs by preventing reuse of freed
memory regions until the garbage collector is able to determine that it is