Fix #79700: Bad performance with namespaced nodes due to wrong libxml assumption

* Use a prepending strategy instead of appending in dom_set_old_ns()

Looping to the end of the list

Fix #79700: Bad performance with namespaced nodes due to wrong libxml assumption

* Use a prepending strategy instead of appending in dom_set_old_ns()

Looping to the end of the list is wasteful. We can just put the new
nodes at the front of the list. I don't believe we can fully prepend,
because libxml2 may assume that the xml namespace is the first one, so
we'll put the new ones as the second one.

* Reuse namespaces from doc->oldNs if possible in dom_get_ns()

* Add a test for reconciling a reused namespace

* Explain why there can't be a cycle between oldNs and nsDef

Closes GH-11376.

Also fixes #77894.

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