We currently do a match on the comparison of every field in a struct or
enum variant. But the last field has a degenerate match like this:
```
match ::core::cmp::Ord::cmp(&self.y, &other.y) {
    ::core::cmp::Ordering::Equal =>
	::core::cmp::Ordering::Equal,
    cmp => cmp,
},
```
This commit changes it to this:
```
::core::cmp::Ord::cmp(&self.y, &other.y),
```
This is fairly straightforward thanks to the existing `cs_fold1`
function.

The commit also removes the `cs_fold` function which is no longer used.

(Note: there is some repetition now in `cs_cmp` and `cs_partial_cmp`. I
will remove that in a follow-up PR.)