[COLLECTIONS-897] Add LexicographicPermutationIterator

[hextriclosan]

Add an Iterator<List> that generates the permutations of a collection in lexicographical order, complementing PermutationIterator, which uses the Steinhaus-Johnson-Trotter ordering.

Elements are ordered by their natural ordering, or by a Comparator supplied to the two-argument constructor, which also allows permuting elements that do not implement Comparable. Each call to next() advances by the standard next-permutation step: locate the pivot, swap it with its successor, then reverse the descending tail. Equal elements are not distinguished, so an input with duplicates yields fewer than n! permutations. An empty collection yields exactly one empty list, as 0! = 1. remove() is unsupported.

Comparator dispatch follows the java.util.TreeMap pattern of testing the comparator field for null on each comparison; benchmarking showed no measurable difference against normalizing null to Comparator.naturalOrder() in the constructor.

Tests extend AbstractIteratorTest to cover the Iterator contract, and add cases for lexicographical exhaustivity, duplicate handling, custom and reverse comparators, non-Comparable elements, stream traversal, exhaustion, and equals/hashCode.

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