Extend blanket Parameterized{En,De}code impls

[tgeoghegan]

Extends the implementation of ParameterizedDecode<()> for D: Decode to
be ParameterizedDecode<T> for D: Decode, T, and similarly extends the
blanket ParameterizedEncode impl. This way, VDAFs like the fake one
in janus that use () as their associated types can still satisfy
bounds like Self::VerifyParam: ParameterizedDecode<Self>. ()
satisfies the trivial trait bound T so this is strictly more
permissive than what was there before.

[tgeoghegan]

The motivation here: Prio3VerifyParam is ParameterizedDecode<Prio3<...>> and Poplar1VerifyParam is ParameterizedDecode<Poplar1<...>>. The fake VDAF in Janus follows that pattern, and its VerifyParam is (), so we need this broader blanket impl so that #55 can do:

impl<A, C> Aggregator<A, C>
where
    A: 'static + vdaf::Aggregator,
    A::AggregationParam: Send + Sync,
    A::VerifyParam: ParameterizedDecode<A>, // <-- this is the trait bound enabled by this change
    for<'a> &'a A::AggregateShare: Into<Vec<u8>>,
    A::PrepareStep: Send + Sync + Encode + ParameterizedDecode<A::VerifyParam>,
    A::OutputShare: Send + Sync + for<'a> TryFrom<&'a [u8]>,
    for<'a> &'a A::OutputShare: Into<Vec<u8>>,
    C: Clock,
{ ... }

I need that trait bound on janus::aggregator::Aggregator so that its constructor can decode a verify param from bytes.