[chore] Optimize `alpha_pow` computation #378

Job	Run time
extension-tests (rv32im, rv32im)	3m 4s
extension-tests (native, native)	3m 13s
extension-tests (keccak256, keccak256)	1m 49s
extension-tests (sha256, sha256)	1m 58s
extension-tests (bigint, bigint)	4m 51s
extension-tests (algebra, algebra)	2m 10s
extension-tests (ecc, ecc)	3m 35s
extension-tests (pairing, pairing)	3m 5s
	23m 45s

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