Describe the bug
Because buf.bit_slice(...) falls back to a slice that ignores the length when the offset is byte-aligned, the resulting buffer has an incorrect length. This leads to problems when applying bitwise operations such as &, for instance.

To Reproduce

The following test fails:

    #[test]
    fn test_bit_slice_aligned() {
        let buf = Buffer::from(&[2, 4, 6, 8, 10]);
        let slice = buf.bit_slice(8, 8);
        let expected = Buffer::from(&[4]);
        assert_eq!(slice.len(), 1); // actual length is 4!!
        assert_eq!(&slice, &expected);
    }

Expected behavior
Test passes.

Additional context
Add any other context about the problem here.