Relax Debug bounds on IpcReceiver and IpcSender

[nox]

Their Debug impls shouldn't require T: Debug.

[antrik]

I think the actual problem is that for some reason deriving Debug on something with PhantomData<T> requires T:Debug? Not sure why it does though -- doesn't make sense to me...

[nox]

No that's only because we derive their implementation, we don't need the bound if we implement it ourselves.

[antrik]

@nox yes -- my point was that I don't see why the derived implementation has to impose these bounds, if a custom implementation doing exactly the same doesn't need it... Unless the derived implementation actually does something else that I'm missing? Otherwise I consider it a shortcoming in Rust, that should be fixed rather than working around it.

[nox]

@antrik It's not really fixable easily in Rust, so we work around it here, nothing too surprising. It's hard for rustc to know which bounds to use for which types.

[nox]

I think a recent nightly does it better, but that means that stable won't derive the same thing for now AFAIK.