feat: add spawn and try_current for madsim-tokio #828

Job	Run time
bench	2m 16s
build	2m 57s
check	2m 6s
doc	1m 16s
test-sim (ubuntu-latest, stable)	4m 19s
test-std (ubuntu-latest, stable)	4m 12s
test-sim (ubuntu-latest, nightly)	4m 34s
test-std (ubuntu-latest, nightly)	4m 17s
test-std (macos-latest, stable)	5m 47s
test-sim (macos-latest, stable)	6m 15s
test-std (macos-latest, nightly)	6m 10s
test-sim (macos-latest, nightly)	7m 8s
	51m 17s

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