There is `slab` (each instance can only store a single type), and `bumpalo` (which allows a mix a types in the same memory chunk). There is also `slotmap` whic

There is `slab` (each instance can only store a single type), and `bumpalo` (which allows a mix a types in the same memory chunk). There is also `slotmap` which isn't really an arena, but can be used for some of the same use cases.