When working with an `object`-backed `memoryview`, it seems we are unable to coerce it to a `list`. This would be useful as it would provide a way to get the underlying `object`'s into something a bit easier to work with.

```
In [1]: import numpy                                                            

In [2]: a = numpy.array(["abc", "def", "ghi"], dtype=object)                    

In [3]: m = memoryview(a)                                                       

In [4]: m.tolist()                                                              
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-4-42400a31add8> in <module>
----> 1 m.tolist()

NotImplementedError: memoryview: format O not supported
```

Can you describe your use-case more? In particular, why use memoryview(a).tolist() instead of a.tolist()? Or some other numpy operations like a.flat or a.view()? Or even numpy.array(memoryview(a)) to recover a numpy array from a memoryview?

If this were implemented and stdlib memoryview objects supported some kind of object 'O' format, would list(memoryview(b"\0\0\0\0\0\0\0\0").cast('O')) dereference a null pointer and segfault?