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Add `Pool::add` #537

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Job	Run time
lint	19s
audit	16s
msrv	28s
coverage	45s
test (ubuntu-latest, stable)	1m 8s
test (ubuntu-latest, beta)	1m 13s
test (macos-latest, stable)	1m 11s
test (windows-latest, stable)	2m 17s
	7m 37s