Improve backward compatability #251

Job	Run time
Book tests	1m 13s
Bitrot check	1m 39s
Build target wasm32-unknown-unknown	1m 36s
Build target wasm32-wasi	1m 33s
Rustfmt	16s
Intra-doc links	1m 16s
Test on ubuntu-latest	6m 53s
Test on i686-unknown-linux-gnu	13m 13s
Test on i686-unknown-linux-gnu with beta features	12m 49s
Test on windows-latest	10m 27s
Test on i686-unknown-linux-gnu with nightly features	14m 51s
Test on macOS-latest	9m 49s
Test on ubuntu-latest with beta features	6m 59s
Test on windows-latest with beta features	10m 24s
Test on macOS-latest with beta features	7m 8s
Test on ubuntu-latest with nightly features	8m 2s
Test on windows-latest with nightly features	10m 53s
Test on macOS-latest with nightly features	7m 50s
	2h 6m 51s

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