Python: impl FromPyObject for reliable FFI

[kylebarron]

I feel like I'd want to expose the buffer protocol on our classes, so that we could call memoryview on any user input and interpret it the same whether it's one of our classes or a plain bytes object. In this case, we might also want to move to top-level functions instead of class methods?

[kylebarron]

We should check if py.extract<Vec<u8>> works

[kylebarron]

We should probably implement FromPyObject on the main index class, but also potentially have a separate "view" class, not exported to Python, so that we can use a python buffer without copying it

[kylebarron]

Closed by #55