Move circuit and its dependencies behind a feature flag #3280

Job	Run time
Latest build on ubuntu-latest	1m 6s
Bitrot check	2m 9s
Book tests	14s
Latest build on windows-latest	2m 4s
Latest build on macOS-latest	56s
Build target wasm32-wasi	1m 26s
Code coverage	9m 0s
Intra-doc links	2m 16s
Rustfmt	15s
Test on ubuntu-latest	10m 15s
Test on windows-latest	11m 45s
Test on macOS-latest	8m 0s
	49m 26s

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