Use DataPayload::from_static_ref in databake

[robertbastian]

This allows inlining the lookup functions and AnyProvider to be backed by DataProvider.

I've also inlined the skeletons special case, as it's only called in one location now and I don't like having the hacky zerocopy on the API.

#2945

[Manishearth]

Any particular reason for killing the lookup method? It feels like it's still neater for diagnostics and debugging.

[Manishearth]

r=me but i kinda liked the separate lookup method, it felt cleaner. We don't have to call it from other spots; it's fine even if it's called just once

(also: the lack of .data.rs for the macro files is kinda annoying, not opposed to that fix being rolled into this PR as well. I'm okay with macros.rs still being .rs)

[robertbastian]

Any particular reason for killing the lookup method? It feels like it's still neater for diagnostics and debugging.

It simplifies adding the fallback (#3487), because we need to put the resolved locale in the metadata. With an inlined #lookup expression it's much simpler, because we can access load's local variables, with a separate fn lookup we'd have to return Result<(&'static #data_struct, Option<DataLocale>), DataErrorKind> which I think gets messy.

[robertbastian]

(also: the lack of .data.rs for the macro files is kinda annoying, not opposed to that fix being rolled into this PR as well. I'm okay with macros.rs still being .rs)

Why is that annoying? They're still marked as generated files.

[Manishearth]

Yeah but you can no longer make them go away completely in the github review view