Performance Improvements

Significant improvements to the runtime of core XPath and XForms functions (path resolution, template contextualization, etc).
Ok, reviewed my history and am confident this was profiled correctly and was a pretty reasonable (10%ish) improvement for some ops.

I think that for things this specific it would be a good policy to justify them more concretely in comments, though, (IE: "Improves performance of FOO by n%"), because it _really_ looks unnecessary.