Function encode 

pub fn encode<'a, I: Iterator<Item = Option<&'a [u8]>>>(
    data: &mut [u8],
    offsets: &mut [usize],
    i: I,
    opts: SortOptions,
)

Variable length values are encoded as

• single 0_u8 if null
• single 1_u8 if empty array
• 2_u8 if not empty, followed by one or more blocks

where a block is encoded as

• BLOCK_SIZE bytes of string data, padded with 0s
• 0xFF_u8 if this is not the last block for this string
• otherwise the length of the block as a u8