Change URL::toString()

## Summary of changes
* Returns single-line output with just the URL
* Masks password with 8 asterisks (********)

## Example
Before

```sh
$ xp -w 'uses("peer.URL"); new URL("http://user:pass@localhost");'
peer.URL@ {
  [scheme]      http
  [host]        localhost
  [port]        0
  [user]        user
  [password]    pass
  [path]
  [query]
  [fragment]
}
```

After

```sh
$ xp -w 'uses("peer.URL"); new URL("http://user:pass@localhost");'
http://user:********@localhost
```