Add Identity::AccountLog

[jazzz]

Adds a raw spec for encoding AccountLogs : a deterministic wire formatting for changes to an account. It focuses on the data format and validation. Distribution and application specific interpretations of entries are not covered.

Context

This spec works towards two different deliverable tracks

AnonComms deliverables

• [Deliverable] Implement basic λAccount library anoncomms-pm#70
• [Deliverable] Specify basic λAccount protocol anoncomms-pm#69,
  Specifically providing: F[1,2,3,4,5,6,7, 9*]

λChat Deliverables

• Implement full identity model logos-messaging/pm#439
  Specifically providing: F[1,2,3,5,6]

Notes

• This spec provides the mechanism to provide EOI via EntryData::Text, but does not define an internal schema.
• F9: "Single-key compromise must not permit irreversible account takeover. " is not strictly satisfied due to the account being bound to a single root signing key which is a single point of failure. With the exception of root key this property holds.
• F1: "An account has a stable, long-lived address that does not change when keys rotate or are revoked." is satisfied as worded, however does not incorporate VLADs at this time due to concerns about memory footprints in post-mainnet implementations.