Move circuit and its dependencies behind a feature flag #3276

Job	Run time
Bitrot check	2m 10s
Book tests	14s
Latest build on ubuntu-latest	57s
Build target wasm32-wasi	1m 28s
Latest build on windows-latest	1m 54s
Latest build on macOS-latest	58s
Code coverage	9m 29s
Intra-doc links	2m 19s
Rustfmt	13s
Test on ubuntu-latest	10m 7s
Test on windows-latest	11m 31s
Test on macOS-latest	9m 32s
	50m 52s

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