Use XPathEvaluators for PT property evaluation

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PTState._eval_prop() should use XPathEvaluator instances instead of the sources' .xpath() method, as this promises to speed things up quite a bit.

Not high on the list, but logging it here so it's not forgotten.

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Should also set up an xpath evaluator on Template (i.e. Template.xpath), as we could avoid the hassle on the processor side of having to use e.g. partial.label.xpath("...", namespaces=partial.nsmap).

Related enough, so I'm not going to split this into two issues. We'll just sort this all out in a single feature branch.