chore: mitigate copies in `final_exponentiation` #123

Job	Run time
Bitrot check	22s
Intra-doc links	8s
Rustfmt	15s
Test on ubuntu-latest	40s
Test on windows-latest	3m 36s
Check wasm target wasm32-unknown-unknown	11s
Test on macOS-latest	38s
Check wasm target wasm32-wasi	11s
	6m 1s

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