Optimize SparsePolynomial arithmetic operations to reduce cloning #3230

Job	Run time
Check no_std	22s
List directories for parallelizing tests	6s
Check (stable)	9s
Check (nightly)	1m 10s
Check Documentation	57s
Check Style	12s
Test (stable)	8m 58s
Test (cp6_782/)	6s
Test (nightly)	17m 34s
Test assembly	8m 19s
Test (mnt4_753/)	11m 9s
Test (mnt6_753/)	12s
Test (./ed_on_bn254)	58s
Test (./ed_on_bls12_381_bandersnatch)	1m 6s
Test (./mnt4_298)	3m 46s
Test (./ed_on_bls12_381)	1m 11s
Test (./ed_on_cp6_782)	1m 14s
Test (./vesta)	1m 10s
Test (./bls12_381)	3m 12s
Test (./ed_on_mnt4_753)	3m 46s
Test (./curve25519)	1m 4s
Test (./pallas)	1m 5s
Test (./ed_on_bw6_761)	41s
Test (./grumpkin)	1m 10s
Test (./secp256r1)	1m 0s
Test (./secp256k1)	1m 5s
Test (./secp384r1)	1m 19s
Test (./bn254)	2m 24s
Test (./bw6_761)	3m 52s
Test (./ed_on_bls12_377)	1m 9s
Test (./bw6_767)	3m 52s
Test (./bls12_377)	4m 3s
Test (./ed_on_mnt4_298)	1m 5s
Test (./secq256k1)	54s
Test (./ed25519)	1m 1s
Test (./mnt6_298)	4m 48s
	1h 36m 9s

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