Improve backward compatability draft2 #383

Job	Run time
Book tests	2m 29s
Build target wasm32-unknown-unknown	2m 50s
Bitrot check	1m 32s
Build target wasm32-wasi	2m 46s
Intra-doc links	2m 32s
Rustfmt	14s
Test on ubuntu-latest	11m 26s
Test on i686-unknown-linux-gnu	21m 20s
Test on windows-latest	15m 4s
Test on i686-unknown-linux-gnu with beta features	21m 28s
Test on macOS-latest	11m 12s
Test on i686-unknown-linux-gnu with nightly features	22m 38s
Test on ubuntu-latest with beta features	11m 13s
Test on windows-latest with beta features	14m 37s
Test on macOS-latest with beta features	10m 33s
Test on ubuntu-latest with nightly features	11m 37s
Test on windows-latest with nightly features	15m 20s
Test on macOS-latest with nightly features	10m 56s
	3h 9m 47s

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