Move circuit and its dependencies behind a feature flag #3281

Job	Run time
Latest build on ubuntu-latest	54s
Latest build on windows-latest	2m 22s
Latest build on macOS-latest	54s
Book tests	15s
Build target wasm32-wasi	1m 34s
Bitrot check	2m 12s
Code coverage	9m 15s
Intra-doc links	2m 13s
Rustfmt	14s
Test on ubuntu-latest	10m 4s
Test on windows-latest	12m 6s
Test on macOS-latest	8m 24s
	50m 27s

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