[generics] type lists in interfaces

[武新飞]

Here is my suggestion to replace "Type lists in interfaces"

```

type Float interface {

    type float32, float64

}

 ```

Can be

```

type Float = float32 | float64

type Ordered = int | int32 | ... Float | string

```

[Ian Lance Taylor]

Thanks for the note.

In the current design draft the section
https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#constraint-type-inference
describes a case where we need both a type list and a method. I don't
see an obvious way to implement that with your suggested syntax.

Ian