Remove Receive 'Msg' type, add actor trait bound

[mkj]

The Msg associated type of Receive trait isn't needed if we require that it's an Actor. It reduces a fair bit of boilerplate. The downside I see is that this forces Receive to be an Actor, though is that a problem?

This is more of a query than a solid pull request, but it's easier to show code examples here than an issue. There might be reasons why it won't work, of course it breaks compatibility too.

[leenozara]

Hi @mkj I do think you're correct - Receive is just Actor. Since this is a change across core code I need to check the overall impact. Will come back to this in the next day or two.

[elenaf9]

@leenozara what is the current status on this?