Constantine backend #47

Job	Run time
tests (ubuntu-latest, blst)	7m 38s
tests (ubuntu-latest, zkcrypto)	53s
tests (ubuntu-latest, arkworks)	9m 17s
tests (ubuntu-latest, constantine)	2m 18s
tests (windows-latest, blst)	3m 46s
tests (windows-latest, zkcrypto)	2m 50s
tests (windows-latest, arkworks)	4m 22s
tests (windows-latest, constantine)	4m 59s
tests (macos-11, blst)	4m 35s
tests (macos-11, zkcrypto)	3m 52s
tests (macos-11, arkworks)	5m 49s
tests (macos-11, constantine)	19m 35s
	1h 9m 54s

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