It's not necessary, since all of these objects are simple data objects
that will be passed as arguments to other functions. When we have to
pass through one of those functions we can just serialize the objects at
that point.
In the host context. So when the plugin wants to create an `IMessage`
object to send a message to the other object, we don't have to go
through the host.
This is quite a huge refactor, but note everything is consistent (and
we're going to need one or two more of these `Vst3*Proxy` objects).
Right now nothing extends `IHostApplication`, but this way it will be
trivial to add support for more host context interfaces.
Robbert van der Helm