Convert _MessageBase to a typealias

[mjburghard]

Remove the _MessageBase protocol in favor of a type alias with different definitions for debug and release builds.
This should decrease the code size without requiring an underscored attribute (see #1497).

[allevato]

This seems like an even better option than the maker protocol.

Since _MessageBase is really just "a list of common protocols that messages conform to", I wonder if it's worth renaming it to hammer that point home. Something like _CommonMessageConformances, so people don't get the idea that they should try to constrain generics based on it or add functionality to it (which fortunately they won't be able to if it's just a typealias).

[mjburghard]

I like the new name. I will rename the type alias.

[tbkka]

Nice!

[thomasvl]

Thanks for the patch!