Cleanup href handling

Remove the Href type since it is not useful in its current form: it is an enum with two variants, but in almost all places where we use it we are asserting that is one of the two. Then replace Fragment with a NodeId enum which is used to acquire nodes.

This partially addresses #622 (closed)

The remaining part involves changing NodeId to hole AllowedUrl instead of a plain String, but to do that we need to first have an additional traversal of the tree where we resolve href using the specified UrlResolver