Safer trait for PyBufferProtocol

[davidhewitt]

At the moment PyBufferProtocol requires users to initialize the raw ffi::Py_Buffer structure in a pointer.

This forces the user to write unsafe code. It would be nice to rewrite this protocol to offer something better.

Prompted by the below discussion:

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In the future I would like to remove Py_buffer::new(), but I think to do so right now would be inconvenient for users because PyBufferProtocol basically forces you to handle the raw ffi pointer (which probably points to uninitialized memory which C created).

I chose MaybeUninit here because that's pretty much identical to what C would do: we create a slab of memory and pass its location to PyObject_GetBuffer to initialize the contents.

Originally posted by @davidhewitt in #1534 (comment)

[kngwyu]

Nice, I also want to implement safe buffer trait for rust-numpy.

[davidhewitt]

This is now tracked in #1884 as part of the move to #[pymethods]