improve safety of Props.create by allowing Creator<T>, see #3377

Props constructors need to be deprecated instead of being mutated
because we cannot just start throwing exceptions in people’s existing
code. Props.withCreator is deprecated for similar reasons, but also
because Props are about the creators, so replacing that after the fact
is not good style.
Merge pull request #1490 from akka/wip-3377-Props.create-∂π

improve safety of Props.create by allowing Creator<T>, see #3377