v0.78.0

SVR-B
-----

- Operations have been consistently renamed to `store` and `restore`.

- `restore` now returns an object containing both the
  BackupForwardSecrecyToken for decryption, and "secret data" to be
  used in the first `store` after restoration.

- `SvrB` now has a `createNewBackupChain` method, allowing you to
  locally persist the backup "secret data" *before* the first store to
  SVR-B for a fresh install.

- With the first two changes, the secret data argument to `restore` is
  now required. See doc comments for more details.

Other Changes
-------------

- Rust: `SessionRecord::has_usable_sender_chain` now takes an
  additional parameter to specify which criteria make a session
  "usable" beyond simply *having* a sender chain. The previous
  behavior can be requested by using
  `SessionUsabilityRequirements::NotStale`.