Scoru,Proto: make [stakers] both a record and private

Context

Having a pair is not ideal because:

1. We use the Index.normalize a lot to ensure that (a = alice, b = bob)
2. The pattern below is very common in the code because of this pair.

let (a, b) = Index.normalize in
let _ = foo (a, b) in
let _ = bar (a, b)

So I propose here to change the pair to a private record, that is, alice is always the correct Staker.t thanks to make. The pair is never reconstructed, we only read from once it's created so (2) is fixed as well.

Manually testing the MR

Checklist

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