add halo2_gadgets_optimized #262

Job	Run time
Book tests	1m 27s
Build target wasm32-unknown-unknown	1m 52s
Bitrot check	1m 28s
Code coverage	21m 46s
Build target wasm32-wasi	1m 39s
Intra-doc links	1m 25s
Rustfmt	24s
Test on i686-unknown-linux-gnu	22m 41s
Test on ubuntu-latest	10m 41s
Test on i686-unknown-linux-gnu with beta features	22m 47s
Test on windows-latest	14m 2s
Test on i686-unknown-linux-gnu with nightly features	26m 55s
Test on macOS-latest	11m 14s
Test on ubuntu-latest with beta features	10m 43s
Test on windows-latest with beta features	14m 56s
Test on macOS-latest with beta features	9m 39s
Test on ubuntu-latest with nightly features	12m 36s
Test on windows-latest with nightly features	17m 46s
Test on macOS-latest with nightly features	11m 33s
	3h 35m 34s

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