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.