feat: optimize verify_msg_block

[TomAFrench]

Description

Problem*

Resolves

Summary*

This PR optimizes verify_block to depend less on mutable values, remove redundant constraints, and remove a println left in in #40

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Changes to circuit sizes

Generated at commit: 6c5aced4250e1816977e4c1462355422f5bfe325, compared to commit: 1100a6638f986159812edb3f2640a262aaa5419a

🧾 Summary (10% most significant diffs)

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
test_sha256_512.json	-2,065 ✅	-27.42%	-4,099 ✅	-7.61%
test_sha256_511.json	-2,054 ✅	-27.31%	-4,087 ✅	-8.20%

---

Full diff report 👇

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
test_sha256_1.json	749 (+16)	+2.18%	36,000 (0)	0.00%
test_sha256_200.json	2,610 (-782)	-23.05%	36,000 (0)	0.00%
test_sha256_512.json	5,465 (-2,065)	-27.42%	49,775 (-4,099)	-7.61%
test_sha256_511.json	5,467 (-2,054)	-27.31%	45,781 (-4,087)	-8.20%