Asks the allocator to shrink the backing allocation to the current __length__, or a capacity
of the user's choosing.
Returns `(did_shrink: bool, err: mem.Allocator_Error)`.
```
shrink(&array) // shrinks to len(array)
shrink(&array, N) // shrink to N capacity
shrink(&map) // shrinks down to len(map)
shrink(&map, N) // shrink to N capacity
```
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