Require `set` #1577

Job	Run time
rubocop	25s
tests (ubuntu-latest, 2.7)	41s
tests (ubuntu-latest, 3.0)	9s
tests (ubuntu-latest, 3.1)	12s
tests (ubuntu-latest, 3.2)	17s
tests (ubuntu-latest, head)	7s
tests (ubuntu-latest, truffleruby-22.2)	37s
tests (ubuntu-latest, truffleruby-22.3)	37s
tests (ubuntu-latest, truffleruby-head)	27s
tests (macos-latest, 2.7)	1m 4s
tests (macos-latest, 3.0)	18s
tests (macos-latest, 3.1)	17s
tests (macos-latest, 3.2)	26s
tests (macos-latest, head)	16s
tests (macos-latest, truffleruby-22.2)	1m 43s
tests (macos-latest, truffleruby-22.3)	1m 31s
tests (macos-latest, truffleruby-head)	44s
	9m 51s

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