Store key derivation function used and iteration count in payload

The KDF used and iteration count should be included in the payload returned from `encrypt` to facilitate upgrades over time. This makes it possible to change the work factor or the algorithm used without breaking backward compatibility.

Library users should be encouraged to upgrade encrypted ciphertexts from time to time to take advantage of this too.

Start at the library's encrypt entry point and trace how the returned payload is structured and how decrypt reads it. Determine how to record the KDF and iteration count while retaining backward compatibility, and verify that the documented upgrade path for older ciphertexts is clear.

Written by the indexing model from the issue text.