Investigate using Clippy and following lints' recommendations

In [this comment](https://github.com/bytecodealliance/regalloc2/pull/1#pullrequestreview-742269939) it was suggested that we run Clippy and follow its lints. It seems that this is somewhat nontrivial as at least some of its lints [seem questionable in our particular case](https://github.com/bytecodealliance/regalloc2/pull/1#issuecomment-909781643) but we should spend the time to look into which particular lints may make sense to enable and clean up our code as necessary.

Review the discussion in pull request #1, especially the linked review and issue comments, to understand which Clippy recommendations are questionable for this register allocator. Run Clippy on the project, assess which lints are appropriate, and clean up the applicable findings. Done means the selected lints are addressed or explicitly justified.

由索引模型根据 Issue 内容生成。