Move circuit and its dependencies behind a feature flag #3295

Job	Run time
Book tests	12s
Bitrot check	2m 6s
Latest build on ubuntu-latest	51s
Latest build on windows-latest	1m 39s
Build target wasm32-wasi	1m 28s
Latest build on macOS-latest	56s
Code coverage	8m 55s
Intra-doc links	2m 14s
Rustfmt	11s
Test on ubuntu-latest	10m 2s
Test on windows-latest	11m 30s
Test on macOS-latest	7m 58s
	48m 2s

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