Follow-up refactoring of `Payload`

[philippecamacho]

The Payload struct has been redesigned (see #1012) however there are some pending improvements including:

• [ ] Get rid of type TxTableEntry See https://github.com/EspressoSystems/espresso-sequencer/blob/dbf68c9a12ec04a0b2b5be1967f7e9b651071efb/sequencer/src/block2/entry.rs#L10
• [ ] This type should be replaced with TableWord with the appropriate trait bounds which will likely involve the use of const generics.
  • [ ] Don't allow arithmetic directly on table entries: zero(), one(), checked_add_mut()
• [ ] Change BlockPayload::Encode to avoid copying the payload.
• [ ] Add unit tests for NameSpaceTable
• [ ] Add unit tests for TxTable
• [ ] Handle errors
• [ ] See if the TxTable / NameSpaceTable can store only a reference to the payload