Allow IActorGrain and GrainWithStringKey in inherited interface

[andyhoyle]

Take the HelloWorld example:

public interface IGreeter : IActorGrain, IGrainWithStringKey { }

public class Greeter : DispatchActorGrain, IGreeter { ... }

I would like to move IActorGrain and IGrainWithStringKey into a base interface:

public interface IBase : IActorGrain, IGrainWithStringKey { }

public interface IGreeter : IBase { }

public class Greeter : DispatchActorGrain, IGreeter { ... }

But when I do this I get an exception:

Warning
System.InvalidOperationException: 'Type 'Demo.Greeter' can only implement single custom IActorGrain interface'

I userstand that when compiled to IL, Greeter gets compiled to:

public class Greeter : DispatchActorGrain, IGreeter, IBase { ... }

But as long as only IGreeter is registered as a reference to the Greeter grain, then there shouldn't be an issue.

I've created a PR to address this: #176