DAP Integration: Prio3 instantiation

[martinthomson]

The current set of Prio instantiations as defined in the VDAF spec do not include any option that is compatible with our proposal.

The closest instantiation has the super awkward name Prio3MultiHotCountVec. That one permits from zero to weight (inclusive) values of 1 in a vector. That is not sufficient as it does not permit values greater than one in any bucket.

Prio3Histogram is further away. It is not even useful when maxValue = 1 because it requires that the sum of values is exactly 1. That makes our opt-out design infeasible.

We will need to develop a mode that sets a per-bucket limit and total weight (also known as an $L_1$ norm) of the same limit. That would combine the checks from Prio3SumVec with the weight check from Prio3MultiHotCountVec.

Note: Using Prio3SumVec constrains maxValue to be $\in [0\dots2^n-1]$, which simplifies the checks. This also simplifies the weight check because it avoids the offset.