`cs_fold` has four distinct cases, covered by three different function
arguments:

- first field
- combine current field with previous results
- no fields
- non-matching enum variants

This commit clarifies things by replacing the three function arguments
with one that takes a new `CsFold` type with four slightly different)
cases

- single field
- combine result for current field with results for previous fields
- no fields
- non-matching enum variants

This makes the code shorter and clearer.