Method add

chapter 5/memory_pool.rs:22–25

(&self, tx: Transaction)

    }

    pub fn add(&self, tx: Transaction) {
        let txid_hex = HEXLOWER.encode(tx.get_id());
        self.inner.write().unwrap().insert(txid_hex, tx);
    }

    pub fn get(&self, txid_hex: &str) -> Option<Transaction> {
        if let Some(tx) = self.inner.read().unwrap().get(txid_hex) {