Replace manual use of index newtypes `bundles[bundle.index()]` with `Index` impls and typed `Vec` wrappers

Right now, regalloc2 uses an entity-component-system sort of pattern with toplevel `Vec`s of `LiveBundle`, `VRegData`, and the like, and newtype'd index wrappers like `LiveBundleIndex`, `VRegIndex`, etc. We have a whole bunch of instances of `self.bundles[bundle.index()]...`.

Ideally we would make `bundles` a `Vec`-wrapper type that has an `Index` implementation that natively takes `LiveBundleIndex`, and then we could make all of these sites slightly less verbose.

在仓库中搜索 self.bundles[bundle.index()] 的出现位置以及类似的手动索引方式，然后检查 LiveBundleIndex、VRegIndex 和相关的索引 newtype。根据这些用法确定带类型的 Vec 包装器和 Index 实现的范围。完成标准是：在不改变行为的情况下，一致地替换已识别出的手动索引转换。

由索引模型根据 Issue 内容生成。