Support registering godot enums and bitfields.

[lilizoey]

In Godot, a class can have integer constants as well as enums/bitfields. These enums and bitfields are largely just a convenient way of grouping together integer constants, since under the hood they are just integer constants and not fully-featured sum types like what rust has. From what i understand, enums and bitfields are largely the same from the perspective of ffi.

We have long had support for registering integer constants associated with a class, when this was added we also made the infrastructure needed to register enums and bitfields with a class. However it wasn't clear what syntax should be used to actually register them with the proc-macros. The most obvious syntax, something like

impl Foo {
  enum Bar { .. }
}

Does not work since enums cannot be declared in an impl block. This would be helped by inherent associated types since we could then declare a type alias in the impl block.

The main things to take into account for enums and bitfields are:

• These enums and bitfields must be registered with a specific class
• They can only contain integer variants

Some possible syntaxes:

#[derive(GodotClass)]
struct Foo { .. }

#[godot_enum_bikeshed_macro(Foo)]
#[repr(u8)]
enum Bar {
  ..
}

#[derive(GodotClass)]
struct Foo { .. }

#[godot_api]
impl Foo {
  #[enum(Bar)]
  const A: u8 = 10;
  #[enum(Bar)]
  const B: u8 = 10;
}

@Houtamelo made a declarative macro that emulates this:

gdscript_rust_enum! {
    GDSCRIPT: TerrainVariant; // Name of the Godot class that will contain the constants
    
    pub enum TerrainVariant {
        Water = 0, // Integer values must be provided for each variant
        Plains = 1,
    }
}