Final slicing in combine_option_bitmap needs to use bit slices

[jhorstmann]

Describe the bug

In combine_option_bitmap the buffer is sliced using Buffer::slice. Since this function is dealing with bitwise validity buffers, it needs to use Buffer::bit_slice instead.

To Reproduce

    #[test]
    fn test_combine_option_bitmap_with_offsets() {
        let bitmap1 =
            make_data_with_null_bit_buffer(8, 1, Some(Buffer::from([0b01010100, 0b1])));
        assert_eq!(
            Some(Buffer::from([0b10101010])),
            combine_option_bitmap(&[&bitmap1], 8).unwrap()
        );
    }

Expected behavior

The above testcase should pass.

Additional context

Bug was probably introduced the combine_option_bitmap was extended to support arbitrarily many buffers.