Factor out vat arithmetic in proof-reuse for MCD #9424

Job	Run time
Version Bump	9s
Code Quality Checks	2m 16s
Profiling	2m 12s
Unit Tests	1m 49s
Build and Test KEVM concrete execution	9m 35s
Nix (normal)	8m 13s
Build and Test KEVM proofs (test-prove-kprove, 45)	4m 52s
Nix (macos-13)	9m 43s
Build and Test KEVM proofs (test-prove-pyk, --no-use-booster, 180)	1s
Nix (ARM64)	4m 42s
Build and Test KEVM proofs (test-prove-pyk, 150)	1s
Build and Test KEVM proofs (test-prove-dss, 10)	11m 36s
	55m 9s

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