Function encode 

pub fn encode<T: FixedLengthEncoding>(
    data: &mut [u8],
    offsets: &mut [usize],
    values: &[T],
    nulls: &NullBuffer,
    opts: SortOptions,
)

Fixed width types are encoded as

• 1 byte 0 if null or 1 if valid
• bytes of FixedLengthEncoding