This affects `runtime.Arena` and `virtual.Arena`, but not currently
`mem.Arena`. These changes allow the last allocation that has been
made to be resized to a larger size by just extending their
allocation in-place, when there's sufficient room in the memory block to
do so.
Shrinking in place and re-using the rest of the allocation can be
supported using almost the same logic, but would require the memory to
be zeroed. Since this would add a additional cost that isn't currently
present, shrinking has not been changed.
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