add test for backward compatibility #250

Job	Run time
Bitrot check	1m 43s
Build target wasm32-unknown-unknown	1m 37s
Intra-doc links	1m 17s
Build target wasm32-wasi	1m 26s
Book tests	1m 42s
Rustfmt	13s
Test on ubuntu-latest	7m 17s
Test on i686-unknown-linux-gnu	12m 51s
Test on windows-latest	9m 49s
Test on i686-unknown-linux-gnu with beta features	13m 1s
Test on macOS-latest	6m 29s
Test on i686-unknown-linux-gnu with nightly features	15m 25s
Test on ubuntu-latest with beta features	7m 5s
Test on windows-latest with beta features	10m 36s
Test on macOS-latest with beta features	6m 36s
Test on ubuntu-latest with nightly features	8m 4s
Test on windows-latest with nightly features	11m 42s
Test on macOS-latest with nightly features	8m 32s
	2h 5m 25s

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