Improve backward compatability #247

Job	Run time
Book tests	1m 22s
Bitrot check	1m 39s
Build target wasm32-unknown-unknown	1m 34s
Test on i686-unknown-linux-gnu	13m 16s
Build target wasm32-wasi	1m 34s
Test on i686-unknown-linux-gnu with beta features	13m 20s
Intra-doc links	1m 16s
Test on i686-unknown-linux-gnu with nightly features	15m 37s
Rustfmt	22s
Test on ubuntu-latest	6m 57s
Test on windows-latest	10m 19s
Test on macOS-latest	13m 8s
Test on ubuntu-latest with beta features	7m 6s
Test on windows-latest with beta features	10m 12s
Test on macOS-latest with beta features	7m 43s
Test on ubuntu-latest with nightly features	7m 56s
Test on windows-latest with nightly features	11m 38s
Test on macOS-latest with nightly features	14m 38s
	2h 19m 37s

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