Split message sending into two (complementary) methods

[JohnMurray]

Summary

Split message sending into two methods:

• Context::send - Utility method that takes ToMessage, performs conversion and calls Context::send_message
• Context::send_message - Takes a Box<Message>, resolves the address, performs serialization/deserialization (in debug mode), and transmits the message.

Motivation

The previous implementation of send_message worked by using ToMessage as a convenience type to cut down on the boilerplate of sending messages. This worked great, however it had one drawback in that is relied on concrete types.

The ability to also send a Box<Message> allows for patterns, abstractions, or middle-men to be built. Take a load-balancer actor for example. It receives Box<Message>s and then forwards them to an actor in a pool. In this instance, there is no concrete type we're dealing with so it is difficult to use ToMessage.

Test Plan

The current examples and tests should cover the new use-case as the new method is still in the critical path.