Implement RaftEntry serialization

Before:

---
- raft_log:
  - +----------+---------+-------+-------------+--------------------------------------+
  - |raft_index|raft_term|raft_id|   command   |                 data                 |
  - +----------+---------+-------+-------------+--------------------------------------+
  - |    1     |    1    |   1   |"EntryNormal"|                  []                  |
  - |    2     |    1    |   1   |"EntryNormal"|[129,164,105,110,102,111,163,70,79,79]|
  - +----------+---------+-------+-------------+--------------------------------------+
...

After:

---
- raft_log:
  - +-------------+-----+-----+------------------------------+
  - | entry_type  |index|term |             msg              |
  - +-------------+-----+-----+------------------------------+
  - |"EntryNormal"|  1  |  1  |          ["empty"]           |
  - |"EntryNormal"|  2  |  1  |["eval_lua","return 2+2 == 4"]|
  - +-------------+-----+-----+------------------------------+
...