Improve backward compatability draft1 #276

Job	Run time
Book tests	1m 36s
Bitrot check	1m 15s
Build target wasm32-unknown-unknown	2m 2s
Build target wasm32-wasi	1m 54s
Code coverage	21m 28s
Test on ubuntu-latest	3m 41s
Intra-doc links	1m 44s
Test on i686-unknown-linux-gnu	3m 42s
Test on i686-unknown-linux-gnu with beta features	3m 49s
Rustfmt	14s
Test on windows-latest	3m 39s
Test on macOS-latest	5s
Test on i686-unknown-linux-gnu with nightly features	3m 49s
Test on ubuntu-latest with beta features	3m 39s
Test on windows-latest with beta features	3m 40s
Test on macOS-latest with beta features	3m 40s
Test on ubuntu-latest with nightly features	3m 35s
Test on windows-latest with nightly features	3m 39s
Test on macOS-latest with nightly features	3m 40s
	1h 10m 51s

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