Nexus.invoke(Request) -> Nexus.request(Request).

This will reduce the potential for compiler confusion when Java 8 allows us to
start writing:

  nexus.invoke(FooEntity.class, foo -> foo.barzle())

because the compiler wouldn't know whether you meant to ignore the return value
(using an Action) or return it (using a Request).

So let's plan now for that glorious eventuality.