Function dictionary_encode 

pub fn dictionary_encode(
    data_block: &DataBlock,
    max_dict_entries: u32,
    max_encoded_size: usize,
) -> Option<(DataBlock, DataBlock)>

Dictionary encodes a data block

Currently only supported for some common cases (string / binary / 64-bit / 128-bit)

Returns a block of indices (will always be a fixed width data block) and a block of dictionary