this keyword as a dynamic return type

related: #68

**example**

You have a fluent interface but one of the method X is slightly restrictive and doesn't return `this` but a subset of its `this`. In this case if the next method Y returns `this` it will share its constraints.

problem: if that method Y may also follow another method Z that returns `this`, its return type becomes dynamic.
solution: introduce a `this` return type

**NB**

It would be impractical to require the user to provide all the possibilities as unions.
It's the job of the static type checker; it should be able to infer the correct type from the current `this`.

**proposal**

```
example(name: Type) => this
```