This PR aligns the `Cells` and `GetCells` message implementations with
the spec: https://github.com/ethereum/devp2p/blob/master/caps/eth.md#getcells-0x14
Previously, `GetCellsPacket` and `CellsPacket` embedded `GetCellsRequest`
and `CellsResponse`. This caused them to be encoded as nested lists, which
does not match the wire format defined by the spec. This PR inlines their fields
to flatten the RLP layout of `Cells` and `GetCells`.