change mux to cond_swap #228

Job	Run time
Bitrot check	1m 38s
Build target wasm32-unknown-unknown	2m 39s
Book tests	2m 8s
Intra-doc links	2m 9s
Rustfmt	11s
Build target wasm32-wasi	2m 22s
Test on ubuntu-latest	8m 1s
Test on i686-unknown-linux-gnu	13m 44s
Test on windows-latest	10m 19s
Test on i686-unknown-linux-gnu with beta features	14m 13s
Test on macOS-latest	8m 34s
Test on i686-unknown-linux-gnu with nightly features	15m 51s
Test on ubuntu-latest with beta features	7m 52s
Test on windows-latest with beta features	10m 13s
Test on macOS-latest with beta features	8m 53s
Test on ubuntu-latest with nightly features	8m 48s
Test on windows-latest with nightly features	11m 30s
Test on macOS-latest with nightly features	15m 45s
	2h 24m 50s

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