Vdaf::prepare_preprocess lacks Vdaf::AggregationParam argument

[tgeoghegan]

Trait Aggregator: Vdaf has:

/// Preprocess a round of preparation shares into a single input to [`Aggregator::prepare_step`].
    fn prepare_preprocess<M: IntoIterator<Item = Self::PrepareShare>>(
        &self,
        inputs: M,
    ) -> Result<Self::PrepareMessage, VdafError>;

However, VDAF-06 defines Vdaf.prep_shares_to_prep(agg_param: AggParam, prep_shares: Vec[Bytes]) -> Bytes. (VDAF). Note the agg_param argument that does not appear in our Rust implementation.

We get away with this because Prio3.prep_shares_to_prep never uses the aggregation parameter. But I'm more confused by the Poplar1 side: the reference implementation of Poplar1.prep_shares_to_prep takes responsibility for decoding field elements from bytes and so needs to know what level of the tree it's on, but libprio's implementation seems to have long since decoded everything. I'm not certain what's going on, but it's difficult to reconcile it with the specification.

Even if there's some clever trick here that makes Poplar1 work out, it seems like crate prio's interface should match up with the specification.