Extract enum variant

Most useful for structs, the tuple case is not so important. E.g.,

```
enum Foo {
    Foo,
    Bar { a: A, b: B },
    Baz(A, B)
}
```

to

```
enum Foo {
    Foo,
    Bar(Bar),
    Baz(Baz)
}

pub struct Bar {
    a: A,
    b: B,
}

type Baz = (A, B);
```

I guess there should be a collapse case too, but that seems less useful. The fun part of this will be getting the pattern matching right.
