Multiple federation protocols: Code restructure

[aumetra]

Restructure the code towards the support for multiple federation protocols

[aumetra]

So the plan for kitsune-core is to literally rip most of the components out of it.
Services go into an own crate, resolvers go into own crates, job definitions go into the job runner crate, etc.

The core crate will pretty much only be there to define traits for the rest of the components, to provide generic interop.

[aumetra]

The kitsune-activitypub crate shouldn't directly reference kitsune-webfinger, but instead something generic that satisfies the Resolver trait, but we aren't there yet where this trait is defined.

[aumetra]

Ugh.. now comes the awkward struggle where I have to somehow share parts of the services crate with the jobs crate in an attempt to use the mailing and user services inside the mailing job..

[aumetra]

I just wanna get this restructure out of my system. It took so much focus (and I suck at focussing at the moment), so I'll just merge when the tests pass, and then test and clean up later.

I don't think I wanna keep the main branch stale for much longer.

[aumetra]

Bloody hell.. not again the stack overflow assertion..
Is it because of the fat pointer needing two machine words instead of two? Is that section really that fragile?