Make as Variant optional for #[messages(...)] attribtue

[hashedone]

The as Variant part of the #[messages(...)] attribute is used only to generate variant names of glue enum, which does not matter in 99.9% of cases - I can only imagine it is relevant when we would have two modules with very similar names producing the same identifier when converted to CamelCase (or maybe the same name when we use path here). And I think that the as Variant is not used by trait impl at all!

We should make the whole as Variant part optional, and if it is missing, we want to build it by converting the module to the CamelCase variant (for path maybe even some name created by CamelCasing all path parts and then merging them with __? I'm unsure if cargo/clippy would complain, but we can always silence this particular warning with #[allow] I think).

Additionally, I would add a warning if as Variant is used with the same identifier as it would be generated from the module name.

[kulikthebird]

According to proc_macro_error warnings emitting is supported only on nightly rust at the moment. The code itself will emit warnings once they're supported in proc macros