Restore code coverage #506

Job	Run time
Book tests	2m 8s
Bitrot check	1m 36s
Build target wasm32-unknown-unknown	2m 25s
Code coverage	2m 20s
Rustfmt	14s
Build target wasm32-wasi	2m 24s
Intra-doc links	2m 9s
Test on i686-unknown-linux-gnu	22m 10s
Test on ubuntu-latest	10m 30s
Test on i686-unknown-linux-gnu with beta features	21m 0s
Test on windows-latest	14m 32s
Test on i686-unknown-linux-gnu with nightly features	21m 36s
Test on macOS-latest	8m 30s
Test on ubuntu-latest with beta features	10m 33s
Test on windows-latest with beta features	14m 59s
Test on macOS-latest with beta features	8m 33s
Test on ubuntu-latest with nightly features	11m 9s
Test on windows-latest with nightly features	14m 24s
Test on macOS-latest with nightly features	9m 20s
	3h 0m 32s

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