Use types instead of records

[greglook]

Some structured types in vault are records, when they really should be types. Switching will improve performance and allow better control of the value properties. For example, a HashID should never need extra keys added to it.

• [ ] vault.blob.digest.HashID
• [ ] vault.entity.datom.Datom

Also evaluate stuff like the blob stores to determine whether they need record semantics as well.