[3.0] [future-work] Cryptographic auth for headers

[bchess]

From @Eta0

Add cryptographic authentication / checksums / ECC (if we're getting fancy) for the entire metadata and header section of the file, as a unit, since this allows checking that the metadata is complete and not missing entries (which could potentially happen if, say, the metadata length segment wasn't set properly at serialization time, or ended up changing somehow).