Change FftError variants to have named members

[WalterSmuts]

Next time a major release is done I think it may be worth it to change the variants of FftError to named members. E.g.

    FftError::InputBuffer(usize, usize)

can be changed to

    FftError::InputBuffer {
        expected_size: usize,
        received_size: usize,
    }

This requires the user to rely less on the documentation.

I don't think this is worth a major version bump on it's own but I do think it's an improvement that's worth the change next time a major version bump is required anyway.

[HEnquist]

This seems like a good idea, and I agree it should wait until the next major version bump. Let's keeps this issue open until then as a reminder.