NonEmpty Slist

Create another data structure similar to `NonEmpty` implemented through the ordinary list. Something like this:

```haskell
data SNonEmpty a = a :| Slist a
```

Start by locating the existing NonEmpty data structure and the ordinary Slist implementation. Compare their public API and tests, then determine the scope needed for an SNonEmpty equivalent to the proposed a :| Slist a representation; done means the new structure is implemented consistently with those existing components.

Written by the indexing model from the issue text.