entry

定义如下，根据是否有交易，可以分为tick和transaction两类entry.

pub struct Entry {

    /// The number of hashes since the previous Entry ID.

    pub num_hashes: u64,//做了多少次hash

    /// The SHA-256 hash `num_hashes` after the previous Entry ID.

    pub hash: Hash,//此次entry的hash大小

    /// An unordered list of transactions that were observed before the Entry ID was

    /// generated. They may have been observed before a previous Entry ID but were

    /// pushed back into this list to ensure deterministic interpretation of the ledger.

    pub transactions: Vec,//交易组成的数组

}