It (1.) is invariant, (2.) must strictly outlive the arena itself,
(3.) constrains the inputs to the arena so that their borrows must
also strictly outlive the arena itself.
This implies that, for now, one can no longer have cross-references
between data allocated via the same `Arena` (even when the data is not
subject to the Drop Check rule). Instead one must carry multiple
`Arena` instances, or (more commonly), use one or more `TypedArena`
instances with enums encoding the different variants of allocated
data.
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