Node-set ordering previously recomputed both nodes' root paths on every
precedes comparison (treePath is O(depth x siblings), so sorts were
O(n log n x depth)). Add decorate-sort-undecorate helpers,
sortedByDocumentOrder() and firstInDocumentOrder(), that compute each node's
order key once, and switch the evaluator's order-and-dedup, the EXSLT set
functions, string-value coercion, and NodeFunctions' target lookup to them.
set:leading/trailing also decorate once, reusing the keys for the pivot
filter and the final sort. precedes stays for pairwise use, sharing the
key comparator (Node.ordered). Behavior unchanged, all suites green.
Closes#113
Mihaela Mihaljevic
b8dbf8e97a