Dereference the Arc before calling methods on the interface

[yonran]

When calling methods on Arc<MyService> where the method is also defined on Arc (e.g. drop and clone), calling handler_copy.method(o, p) will actually attempt to call the Arc method instead of the method on the service, resulting in a compiler error. This change removes the ambiguity by dereferencing the Arc.

// proto file:
message DropReq {}
message DropResp {}
service HelloService {
    rpc Drop (DropReq) returns (DropResp);
    rpc Clone (DropReq) returns (DropResp);
}

// Generated _grpc.rs file:
// server
impl HelloServiceServer {
    pub fn new_service_def<H : HelloService + 'static + Sync + Send + 'static>(handler: H) -> ::grpc::rt::ServerServiceDefinition {
        …
                        ::grpc::rt::MethodHandlerUnary::new(move |o, p| handler_copy.drop(o, p))
        // compiler error: explicit destructor calls not allowed
        // compiler error: expected 0 parameters
        // compiler error: expected struct `grpc::resp::SingleResponse`, found ()
    }
}

An alternative fix would be to use the fully qualified syntax for method calls (yonran@d5d7f4c). I believe that this change should be good enough because H is only known to be H : HelloService + 'static + Sync + Send + 'static, and Sync and Send define no other methods.

[stepancheg]

Merged as aafba06.

Thanks!