divviup_client::task::Histogram should have `length()` and `chunk_length()` methods

[inahga]

Suppose a collector is using Divvi Up for task discovery. It needs to make Vdaf instantiations which require the length and chunk_length. To get these values, it needs to match each variant of Histogram to extract these values, something like this:

            DivviupVdaf::Histogram(histogram) => match histogram {
                Histogram::Categorical {
                    buckets,
                    chunk_length,
                } => Self::Prio3Histogram {
                    length: buckets.len(),
                    chunk_length: chunk_length
                        .ok_or_else(|| anyhow!("chunk_length must be provided for DAP-09 tasks"))?
                        .try_into()?,
                },
                Histogram::Continuous {
                    buckets,
                    chunk_length,
                } => Self::Prio3Histogram {
                    length: buckets
                        .len()
                        .checked_add(1)
                        .ok_or_else(|| anyhow!("overflow when calculating histogram length"))?,
                    chunk_length: chunk_length
                        .ok_or_else(|| anyhow!("chunk_length must be provided for DAP-09 tasks"))?
                        .try_into()?,
                },
                Histogram::Length {
                    length,
                    chunk_length,
                } => Self::Prio3Histogram {
                    length: length.try_into()?,
                    chunk_length: chunk_length
                        .ok_or_else(|| anyhow!("chunk_length must be provided for DAP-09 tasks"))?
                        .try_into()?,
                },
            },

It would be better if these were methods on Histogram instead, to avoid having to avoid the user needing to do the non-obvious matching logic themselves.

[tgeoghegan]

I believe I went and did this in #1019