add a section to compare two lookup arguments #256

Job	Run time
Bitrot check	1m 31s
Build target wasm32-unknown-unknown	1m 40s
Book tests	1m 18s
Build target wasm32-wasi	1m 35s
Intra-doc links	1m 23s
Rustfmt	15s
Test on i686-unknown-linux-gnu	13m 31s
Test on ubuntu-latest	7m 10s
Test on i686-unknown-linux-gnu with beta features	13m 15s
Test on i686-unknown-linux-gnu with nightly features	14m 49s
Test on windows-latest	10m 40s
Test on macOS-latest	13m 44s
Test on ubuntu-latest with beta features	7m 13s
Test on windows-latest with beta features	10m 53s
Test on macOS-latest with beta features	8m 34s
Test on ubuntu-latest with nightly features	8m 8s
Test on windows-latest with nightly features	11m 29s
Test on macOS-latest with nightly features	8m 10s
	2h 15m 18s

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