Missing type parameter for `PhantomData`

[shniubobo]

If a struct's type parameter is only used in a PhantomData field, it is missing from the generated .d.ts.

To reproduce

Clone this repo and follow the steps in README.md, which generates:

/* tslint:disable */
/* eslint-disable */
export interface Foo {}

... from:

pub struct Foo<T> {
    bar: PhantomData<T>,
}

Expected behaviour

Either of the following should be generated:

• /* tslint:disable */
  /* eslint-disable */
  export interface Foo<T> {}

• /* tslint:disable */
  /* eslint-disable */
  export interface Foo<_> {}

[shniubobo]

It seems that the omitted type parameter is required by enums with generics, which I don't fully understand for now, but I guess it's safe to keep these parameters on structs and prefix them with a underscore. I've implemented that at shniubobo@52991f8 as a temporary workaround for myself, and I'm not sure whether that is suitable for PR. Maybe we could discuss the desired behaviour?