MessageHandler to replace service_list! macro

[dymk]

• Replace service_list! macro with MessageHandler, which provides an API that makes it more obvious what is happening instead of hiding the "build a static linked list" magic behind a macro.
• Make UUID and NAME constants on the Service trait, as they shouldn't be dynamically computed values
• Rename RegisterPayload to DirectMessagePayload to make it more clear that it's not just the raw register values x0... from the incoming FFA message (it's x4... onwards).
• Remove indirection with the Payload trait which wasn't adding any value other than making the direct message types themselves be directly usable as their message payload... which didn't actually make the code using that functionality any cleaner.

[philgweber]

Looks good.