Ledger format improvements

[jumaffre]

This issue tracks future improvements to the ledger format.

1. Follow-up from #5331. To be able to serialise the snapshot directly into host memory, the hash of the snapshot should be able to be computed as it is serialised, rather than being serialised first and then hashed. To do so, the GCM header should be placed at the end of the serialised snapshot, rather than at the start, as it is only computed once all public and private domains have been encrypted/integrity-protected. We could make this change for all transactions too, to reduce the cost of serialising and then hashing each one.