feat: optimize bitshifts

[TomAFrench]

Description

Problem*

Resolves

Summary*

This PR removes some overuse of lshift8 and optimizes lshift8 with usage of a lookup table.

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: f7b683c9158237ce9927bfbf71d29a70192b2ad9, compared to commit: 5fb54c8d86d83eb62279ac78b7d246f22a66b75a

🧾 Summary (10% most significant diffs)

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
test_sha256_200.json	+242 ❌	+7.52%	-351 ✅	-1.23%
test_sha256_1.json	-138 ✅	-14.71%	-583 ✅	-4.43%

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Full diff report 👇

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
test_sha256_512.json	7,442 (+850)	+12.89%	53,286 (+27)	+0.05%
test_sha256_511.json	7,587 (+850)	+12.62%	49,494 (+24)	+0.05%
test_sha256_200.json	3,458 (+242)	+7.52%	28,095 (-351)	-1.23%
test_sha256_1.json	800 (-138)	-14.71%	12,591 (-583)	-4.43%