Clean imports #462

Job	Run time
Book tests	1m 47s
Build target wasm32-unknown-unknown	2m 6s
Bitrot check	1m 37s
Build target wasm32-wasi	1m 59s
Intra-doc links	1m 46s
Rustfmt	14s
Test on i686-unknown-linux-gnu	22m 54s
Test on i686-unknown-linux-gnu with beta features	22m 19s
Test on ubuntu-latest	10m 49s
Test on windows-latest	13m 50s
Test on macOS-latest	8m 32s
Test on i686-unknown-linux-gnu with nightly features	22m 46s
Test on ubuntu-latest with beta features	10m 52s
Test on windows-latest with beta features	14m 33s
Test on macOS-latest with beta features	8m 43s
Test on ubuntu-latest with nightly features	11m 5s
Test on windows-latest with nightly features	15m 22s
Test on macOS-latest with nightly features	8m 35s
	2h 59m 49s

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