add halo2_gadgets_optimized #264

Job	Run time
Book tests	1m 29s
Bitrot check	1m 32s
Build target wasm32-unknown-unknown	1m 47s
Build target wasm32-wasi	1m 43s
Test on ubuntu-latest	10m 44s
Test on windows-latest	13m 59s
Test on i686-unknown-linux-gnu	21m 49s
Code coverage	21m 43s
Test on macOS-latest	18m 54s
Test on i686-unknown-linux-gnu with beta features	22m 23s
Test on ubuntu-latest with beta features	10m 38s
Test on i686-unknown-linux-gnu with nightly features	27m 57s
Test on windows-latest with beta features	14m 34s
Intra-doc links	1m 27s
Test on macOS-latest with beta features	11m 22s
Test on ubuntu-latest with nightly features	12m 52s
Rustfmt	16s
Test on windows-latest with nightly features	17m 22s
Test on macOS-latest with nightly features	11m 35s
	3h 44m 6s

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