Make Var use GodotConvert::Via to align it with ToGodot and FromGodot

[lilizoey]

Var now depends on GodotConvert and uses GodotConvert::Via instead of its own intermediate type

Var does not depend on ToGodot or FromGodot to make it possible to have properties that cannot be used as function arguments/return types. This is currently the case for OnReady<T>. The current implementations do however use ToGodot and FromGodot to perform the conversions.

This also required some changes in the GodotConvert and Var macros to make sure they agree on the Via type to use for structs/enums.

I didn't change the derive macros much beyond making them compatible with each other, as i think they could do with a bigger rewrite later, see for instance #452.

closes #453

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-595

[Bromeon]

Thank you! Nice to see that some parts get even simpler 🙂

[Bromeon]

I don't think this is unreachable -- a user could still specify #[repr = u32] or something 🤔

So might be better to bail here...

[lilizoey]

any form of #[repr] that doesn't look like #[repr()] will give a rust compile time error:

#[repr = i64] // attribute value must be a literal
#[repr = "i64"] // malformed `repr` attribute input
#[repr] // malformed `repr` attribute input
#[repr(i64, i32)] // conflicting representation hints, this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!

however #[repr()] is just a warning apparently:

#[repr()] // unused attribute, attribute `repr` with an empty list has no effect 

which surprised me a bit tbh, i'll fix that then ig,

[Bromeon]

This is the only thing left, otherwise I think it's ready 🙂

[Bromeon]

Also here, what if the user specifies #[repr()] or #[repr(+)]?

[lilizoey]

#[repr(+)] // expected unsuffixed literal or identifier, found `+`

[Bromeon]

Thank you!