SLL

Migrated to SLL - no higher-order functions. As a result, analysis is much more simplified. However, in this version it is evident that the lack of nested case expressions is bad, - original `foetus` is limited to infer `=` in  (for example) this case.

```
f(S(x)) = g(S(x))
```

There is an idea how to tangle it with supercompilation-like `foetus`.