Factor out vat arithmetic in proof-reuse for MCD #9409

Job	Run time
Version Bump	6s
Code Quality Checks	2m 0s
Build and Test KEVM concrete execution	16m 14s
Unit Tests	1m 50s
Profiling	2m 23s
Nix (normal)	8m 8s
Build and Test KEVM proofs (test-prove-kprove, 45)	3m 35s
Nix (macos-13)	25m 45s
Build and Test KEVM proofs (test-prove-pyk, --no-use-booster, 180)	4m 49s
Nix (ARM64)	4m 51s
Build and Test KEVM proofs (test-prove-pyk, 150)	1m 48s
Build and Test KEVM proofs (test-prove-dss, 10)	4m 54s
	1h 16m 23s

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