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The arena, a fast but limited type of allocator.
Arenas are a type of allocator that destroy the objects within, all at
once, once the arena itself is destroyed. They do not support deallocation
of individual objects while the arena itself is still alive. The benefit
of an arena is very fast allocation; just a pointer bump.
This crate implements `TypedArena`, a simple arena that can only hold
objects of a single type.