Unify field extension arithmetic #484

Job	Run time
Bench (default)	3m 52s
Rustfmt	20s
Clippy lint checks	42s
Wasm-compatibility (wasm32-unknown-unknown)	2m 19s
Wasm-compatibility (wasm32-wasi)	2m 13s
Test (default)	5m 0s
Bench (asm)	5m 7s
Spell Check with Typos	7s
Test (bn256-table)	4m 55s
Bench (bn256-table)	5m 18s
Test (derive_serde)	5m 8s
Test (asm)	4m 41s
	39m 42s

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