Fix the Atom feed generator

I wasn't able to read the Atom feed produced by http://planet.factorcode.org/ using the `syndication` vocabulary, so I tried to fix that issue. Here is a simple test case which currently fails with an error:
```factor
USING: syndication xml ;
"vocab:syndication/test/atom.xml" file>xml xml>feed feed>xml xml>feed
```
The problem is that `feed>xml` generates an ill-formed Atom feed: indeed, `link` tags miss the required `rel="alternate"` attribute.
This pull request adds missing attributes and updates corresponding unit tests to check whether the `feed>xml xml>feed` conversion chain results in the same feed.