add main function #208

Job	Run time
Bitrot check	1m 46s
Book tests	1m 32s
Build target wasm32-unknown-unknown	1m 57s
Build target wasm32-wasi	1m 50s
Code coverage	21m 31s
Intra-doc links	1m 33s
Rustfmt	13s
Test on ubuntu-latest	7m 20s
Test on windows-latest	9m 51s
Test on i686-unknown-linux-gnu	13m 27s
Test on macOS-latest	16m 42s
Test on i686-unknown-linux-gnu with beta features	13m 27s
Test on ubuntu-latest with beta features	7m 27s
Test on i686-unknown-linux-gnu with nightly features	15m 37s
Test on windows-latest with beta features	9m 40s
Test on macOS-latest with beta features	12m 30s
Test on ubuntu-latest with nightly features	8m 22s
Test on windows-latest with nightly features	11m 1s
Test on macOS-latest with nightly features	12m 19s
	2h 48m 5s

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