Use Ref for generic type parameters

[kennykerr]

Now that Ref no longer requires a lifetime parameter #3433 we can use it in a trait as a type alias and generalize the concept of a reference or input parameter for generic WinRT types. This solves a longstanding problem with generic type parameters where they were generated as references "just in case" since there was no way to know whether it would be specialized with a copyable or non-copyable type.

So now whether you're implementing specializations of IVector<T> or writing a closure for TypedEventHandler<Sender, Result>, you can directly use the generic type arguments without having to indirect through a reference. For generic implementations - implementations that have to deal with any kind of T - you can use the Type::deref helper to convert a T::Ref into a &T::Default for efficient but generic handling of type parameters.

This also gets us closer to resolving #3233 and #1339.