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feat: add filter to HashMap, HashSet and Set, and retain to Set (#4255)

Follow-up to #4253/#4254, which added Map::filter.

- HashMap::filter, HashSet::filter and Set::filter return a new
  collection holding the elements the predicate keeps. Like
  Map::filter, they reuse each entry's stored hash (no rehashing, no
  key comparisons, so no Hash/Eq bound), size the result the way each
  type's constructor would, and accept a raising predicate. Set::filter
  keeps insertion order.
- Set::retain: Set was the only one of the four with no in-place
  filter. It mirrors Map::retain, and the shift_back safety comments now
  cover its stored-index route the same way Map's do.
- The retain docstrings for HashMap and HashSet now say that true means
  keep and point to filter, as Map's does.

On 50,000 String keys (native, release), filter is 2.3-2.6x faster than
rebuilding from a comprehension; with Int keys, 1.5x for HashMap and
1.1-1.2x for the two sets, which build no tuples.

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

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