Improve backward compatability draft1 #281

Job	Run time
Build target wasm32-unknown-unknown	2m 18s
Test on ubuntu-latest	3m 52s
Build target wasm32-wasi	2m 12s
Test on windows-latest	3m 58s
Test on i686-unknown-linux-gnu	37s
Test on macOS-latest	1s
Test on i686-unknown-linux-gnu with beta features	36s
Test on ubuntu-latest with beta features	4m 1s
Test on i686-unknown-linux-gnu with nightly features	45s
Test on windows-latest with beta features	3m 58s
Test on macOS-latest with beta features	3m 59s
Test on ubuntu-latest with nightly features	3m 58s
Test on windows-latest with nightly features	3m 58s
Test on macOS-latest with nightly features	3m 59s
Bitrot check	1m 12s
Book tests	1m 58s
Code coverage	21m 26s
Intra-doc links	2m 5s
Rustfmt	26s
	1h 5m 19s

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