Apply rustfmt-style formatting

[alonmuroch]

Summary

• reorder decoder imports in cpu.rs
• format instruction.rs match arms
• clean up trailing whitespace and blank lines
• reorder module declarations in lib.rs

Testing

• cargo check
• cargo test

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https://chatgpt.com/codex/tasks/task_e_6845a6b76fb8832386592660f27b6d23