build: disable unnecessary features of `tokio` #326

Job	Run time
Checking fmt and docs	17s
clippy_check	13s
Build and test (ubuntu-latest, nightly)	30s
Build and test (ubuntu-latest, beta)	30s
Build and test (ubuntu-latest, stable)	19s
Build and test (windows-latest, nightly)	9s
Build and test (windows-latest, beta)	18s
Build and test (windows-latest, stable)	8s
Build and test (macOS-latest, nightly)	30s
Build and test (macOS-latest, beta)	32s
Build and test (macOS-latest, stable)	2s
	3m 28s

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