explore using libxml2's xmlReconciliateNs() to deal with reparenting

[flavorjones]

Currently Nokogiri tries to figure out namespaces after node reparenting via our homebrewed function node.c:relink_namespace(), however what we're doing there is probably not quite right as evidenced by the number of issues as well as the commentary on a few PRs, notably #2495 and #2310.

Recently I discovered that libxml2 has a method, xmlReconciliateNs(), which seems to do what we want (from a brief reading).

This issue is to explore using xmlReconciliateNs() with careful attention paid to:

• does this fix some of the issues reported
• does this break existing tests
• does this break downstream consumers of Nokogiri

[nwellnhof]

xmlReconciliateNs isn't used much. It has a weird way to resolve namespace clashes and is still buggy.

[flavorjones]

@nwellnhof Thanks, I just came back to this issue to link to your comment from #613.

It's still something I'd like to explore -- I'm not at all convinced that what Nokogiri is doing today is better. And if I discover edge cases I might be able to help contribute to an updated/new implementation.

[nwellnhof]

Another option, but probably even less tested, is xmlDOMWrapAdoptNode. This function seems to implement W3C DOM Namespace Normalization and should be called before moving a node. There's also xmlDOMWrapReconcileNamespaces.