Compress TransactionUpdate

[jeremybanka]

TransactionUpdate is a lot bulkier than it has to be:

type TransactionUpdate = {
  id: string // specific to this transaction instance
  key: string // common to the transaction implementation
  epoch: number // serial number in continuity
  updates: (AtomUpdate | TransactionUpdate)[]
}
type AtomUpdate<T> = {
  key: string
  family?: {
    key: string
    subKey: Json.Serializable
  }
  newValue: T
  oldValue: T
}

This can be flattened:

type TransactionUpdate = [
  begin: "<$transaction_update>"
  epoch: number,
  id: string,
  key: string,
  ...([...TransactionUpdate] | [...AtomUpdate])[]
  close: ""
]
type AtomUpdate<T> = [
  type: "<a>"
  key: (string | [key: string, familyKey: string, familySubKey: Json.Serializable]),
  oldValue: T,
  newValue: T,
  type: "</a>"
]