1. When the arena is already empty, resetting with `retain_capacity` no longer
results in allocating a buffer with zero capacity.
This behavior was previously intended by the `(current_capacity == 0)` check,
but wasn't correctly implemented.
2. Resetting with `.{ .retain_with_limit = 0 }` is now equivalent to
`free_all` and a new buffer with zero capacity is no longer created.
This is a useful side-effect of the above fixes.
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