Mismatching types between divviup-client and libprio-rs/janus

[inahga]

In divviup-client, we have types that don't cleanly match those of the janus and libprio-rs crates.

For instance, in Vdaf we have length and bits parameters that need to be converted to usize for use in libprio-rs. Ideally the divviup-client representations would match those of janus and libprio-rs, so the user doesn't need to handle .into()/.try_into()/as conversions.

[inahga]

Alternatively we could more closely consider the use cases that users may have for directly using parameters, besides displaying. Maybe it's more useful to have a impl TryInto<prio::vdaf::Collector> for divviup::client::Vdaf that does all integer conversions for us.