use AsRef in cut_through for flexibility

[antiochp]

Make cut_through() more idiomatic, making it significantly more flexible in terms of what we can do with it.
It now takes two slices of anything that is AsRef<Commitment>.
So we can cut_through inputs and outputs.
We can also cut_through inputs and output identifiers.
We could also (if we wanted) cut_through inputs and inputs, finding the intersection and difference in the process.

Also took the opportunity to refactor finding the total len() of all inputs, outputs, kernels in aggregate() using fold() to remove some mut definitions.