feat(tokio_util): Stabilise JoinMap #11139

Job	Run time
fmt	12s
clippy	34s
minrust	20s
Check README	2s
docs (windows-latest)	23s
docs (ubuntu-latest, --cfg tokio_taskdump, --cfg tokio_taskdump)	22s
basic checks	1s
test tokio full	1s
test all crates in the workspace with all features	1s
check tokio full --internal-mt-counters	1s
Run integration tests for each feature	1s
compile tests with parking lot send guards	1s
test tokio full --unstable	1s
test tokio full --unstable --taskdump	1s
test all crates in the workspace with all features and panic=abort	1s
valgrind	1s
asan	1s
cross-check	1s
cross-check-tier3	1s
cross-test-without-parking_lot	1s
cross-test-with-parking_lot	1s
miri-doc	1s
miri-lib	1s
miri-test	1s
semver	1s
check-external-types (${{ matrix.os }})	1s
check-fuzzing	1s
build tokio for redox-os	1s
check unexpected lints and cfgs	1s
features ${{ matrix.name }}	1s
build loom tests	1s
minimal-versions	1s
Check tokio --feature-powerset --depth 2 on i686-unknown-linux-gnu without AtomicU64	1s
Test tokio --all-features on i686-unknown-linux-gnu without AtomicU64	1s
Test hyper	1s
Test Quinn	1s
test tokio for wasm32-unknown-unknown	1s
${{ matrix.target }}	1s
build tokio for x86_64-fortanix-unknown-sgx	1s
check-spelling	1s
all systems go	1s
	2m 28s

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