Is `A: Arbitrary` bound needed for `PhantomData<A>`?

[Jangseop Shin]

It prevents me from deriving Arbitrary from struct like the following:

#[derive(Arbitrary)]
struct MyStructPointer<'a> {
  ...
  _lifetime: PhantomData<&'a MyStruct>,
}

It works fine after removing the bound.

[Manish Goregaokar]

We should introduce a blanket impl