Improve backward compatability #253

Job	Run time
Bitrot check	1m 38s
Build target wasm32-unknown-unknown	1m 36s
Build target wasm32-wasi	1m 26s
Test on ubuntu-latest	7m 0s
Test on i686-unknown-linux-gnu	12m 55s
Test on windows-latest	10m 27s
Test on i686-unknown-linux-gnu with beta features	13m 4s
Test on macOS-latest	12m 19s
Test on i686-unknown-linux-gnu with nightly features	15m 40s
Test on ubuntu-latest with beta features	7m 10s
Test on windows-latest with beta features	10m 10s
Test on macOS-latest with beta features	6m 43s
Book tests	1m 14s
Test on ubuntu-latest with nightly features	8m 5s
Test on windows-latest with nightly features	11m 49s
Test on macOS-latest with nightly features	8m 43s
Intra-doc links	1m 17s
Rustfmt	15s
	2h 11m 31s

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