Allow defining inline structs in diagnostics!

[ada4a]

Just a random TODO comment I thought I'd implement. I have some doubts about this though:

• is this still desired?
• is removing pub and struct really worth it? I feel like one'd write them almost instinctively and be surprised when it doesn't work. Maybe I could adjust the macro to look for $($($vis:vis)? strukt:ident)? instead?
• the macro definition ended up being somewhat cursed
• formatting is completely broken

Also, the diff is pretty hard to review, sorry about that. I found that commenting out IncorrectCase helped a little bit, hence the second commit that uncomments it.

[ChayimFriedman2]

I personally think the old way is fine, and don't want this change.

[lnicola]

Not sure about the complexity, but that enum is a bit weird, do we still want it to be an enum in the long run?