fn divisor_for_capacity(capacity: usize) -> u64 {
            let capacity: u64 = capacity.try_into().expect("usize exceeds u64");
            if capacity == 0 || capacity == 1 { 0 }
            else {
                ((1 << 63) / capacity) << 1
            }
        }
A value large enough that any `u64` divided by it is less than `capacity`.

This is `2^64 / capacity`, except in the cases where `capacity` is zero or one.
In those cases, we'll return `0` to communicate the exception, for which we should
just return `0` when announcing a target location (and a zero capacity that we insert
into becomes a bug).
differential-dataflow/src/trace/implementations/rhh.rs