Prio3SumVec: Use Field64

As of draft 12, Prio3Sum uses Field64 instead of Field128, since we no
longer use joint randomness in the circuit.

src/vdaf/prio3.rs

@@ -87,7 +87,7 @@ impl Prio3Count {
 /// The count-vector type. Each measurement is a vector of integers in `[0,2^bits)` and the
 /// aggregate is the element-wise sum.
 pub type Prio3SumVec =
-    Prio3<SumVec<Field128, ParallelSum<Field128, Mul<Field128>>>, XofTurboShake128, 16>;
+    Prio3<SumVec<Field64, ParallelSum<Field64, Mul<Field64>>>, XofTurboShake128, 16>;
 
 impl Prio3SumVec {
     /// Construct an instance of Prio3SumVec with the given number of aggregators. `bits` defines