feat: add timer support for `legacy::Pool` #278

Job	Run time
Check Style	11s
Check MSRV (1.63)	1m 48s
Test with Miri	48s
features	49s
Test stable on ubuntu-latest	35s
Test stable on windows-latest	1m 42s
Test stable on macOS-latest	1m 10s
Test beta on ubuntu-latest	37s
Test beta on windows-latest	1m 19s
Test beta on macOS-latest	1m 22s
Test nightly on ubuntu-latest	34s
Test nightly on windows-latest	1m 31s
Test nightly on macOS-latest	1m 21s
Build docs	20s
CI is green	1s
	14m 8s

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