Argon2: Precompute or cache 2i/2id indices

[solardiz]

As I wrote in #2738 (comment)

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... potential optimization for 2i and 2id, where the data-independent indices can be reused across hash computations, rather than recomputed each time like upstream does. In our current hack of older upstream code, we're already passing the pseudo_rands array from the application, yet somehow we don't appear to be making this optimization. In latest upstream code, this array is gone - as far as I can tell, the data-independent indices are being calculated in smaller portions, which makes sense for that approach - but we'll probably need to reintroduce an equivalent of the array (as an option), to be written-to (if for the first time or when invoked with higher parameters) and reused (on subsequent calls with same or lower parameters) where the new upstream code normally does these things (so that we won't deviate from upstream too much). This probably means in next_addresses().

I think https://gitlab.com/omos/argon2-gpu already has this optimization (for GPU) - grep it for "precompute".
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However, now that we got a revision of the above OpenCL implementation into our tree here, it doesn't appear to have this optimization. Maybe it was dropped at some point, or maybe I was wrong that it was there, or am wrong that it isn't now. But right now it looks to me that we'd need to try implementing this for both CPU and OpenCL.

The pseudo_rands array is indeed gone from our tree with #5557.