Fix Argon2 multiple lanes bug

Fix Argon2 multiple lanes bug

Argon2 failed to conform to the reference implementation when used with
multiple lanes, rendering it useless for this compatibility use case.
The error came from the way we select the reference set:

- On the first slice of the first pass, only the current lane is valid.
- When selecting other lanes, only fully completed segments are valid.
- The previous block of *all* lanes must be excluded.

Fixes #263