BYO signing sorter impl. (#447)

* Bring your own signing sort impl. Allows consumers to e.g. use Rayon for faster multi-threaded sorting instead of the default slow single-threaded sorting.
* Also, don't insert into a self-sorting collection while collecting NSEC3s, instead push to an unsorted vec then sort before iterating, as post-sorting is much faster.

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Co-authored-by: Jannik Peters <[email protected]>