PERF: Cythonize `from_nested_dict`
I did try to cythonize this function, but the problem is that it also accepting a ```collections.OrderedDict```, which cannot be ```cdef``` as ```dict```, but have to ```cdef``` as ```object```.

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I don't see much of a performance boost, I have ran the full benchmark suite and ```asv``` says that the ```BENCHMARKS NOT SIGNIFICANTLY CHANGED.```

Maybe just better to remove the ```TODO``` note, thoughts?