Improve sugar for newtype
Improve sugar for newtype

Now formats e.g.
```
let f (type v) (x: v) = x
```
to itself instead of
```
let f = fun (type v) -> fun (x: v) -> x
```

close #54

ocaml-ppx/ocamlformat,ocaml-ppx/ocamlformat,ocaml-ppx/ocamlformat