Add `Sendable` to more places that folks might want it. #894

Job	Run time
core (5.8.1-jammy, ref/heads/main)	1m 19s
core (5.7.3-jammy, ref/heads/main)	1m 4s
core (5.6.3-focal, ref/heads/main)	52s
fuzzing_regressions (debug)	43s
fuzzing_regressions (release)	39s
sanitizer_testing (address, debug)	1m 3s
sanitizer_testing (address, release)	49s
sanitizer_testing (thread, debug)	1m 0s
sanitizer_testing (thread, release)	1m 1s
	8m 30s

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