mod ipred: Use wrap_fn_ptr! to generate decl_fn!s and add type-safe wrappers

[kkysen]

This also reworks wrap_fn_ptr! to declare Fn as a newtype struct inside the macro rather than a type alias to the generic WrappedFnPtr, which needed a type token for disambiguation. This results in mode code inside the macro vs. out of it, but the code is much simpler and easier to understand this way.

[rinon]

What is the WrappedFnPtr type actually giving us here? It would be simpler to make Fn itself a newtype instead of making it a pub type. That would avoid the Token thing entirely, right?

[kkysen]

What is the WrappedFnPtr type actually giving us here? It would be simpler to make Fn itself a newtype instead of making it a pub type. That would avoid the Token thing entirely, right?

Yeah, the alternative to a token type is to just make a newtype, but since most of the rest besides that token type part can be a generic type, I thought it's easier that way as less code has to be inside the macro.

[rinon]

The Token type is hard to understand, IMO. It took me a bit to figure out what the actual problem was, because I forgot that we were even using a pub type instead of a fresh newtype. I think the newtype is the way to go here, whether the code is inside or outside the macro doesn't matter here because we don't use WrappedFnPtr elsewhere.

[kkysen]

The Token type is hard to understand, IMO. It took me a bit to figure out what the actual problem was, because I forgot that we were even using a pub type instead of a fresh newtype. I think the newtype is the way to go here, whether the code is inside or outside the macro doesn't matter here because we don't use WrappedFnPtr elsewhere.

I tried out declaring the newtype inside the macro and it is indeed a lot cleaner and simpler.