Add support for C unions

[HertzDevil]

C unions are distinct from Crystal Unions for a few reasons:

• C unions are a layout constraint that associates names to each variant member; Crystal Unions are a type constraint and cannot do the same.
• C unions may use repeated types among variant members, like button and axis below.
• It is well-defined in C to write to a variant member of a union and then read from a different variant member. (This is undefined behaviour in C++ but most compilers use the C behaviour anyway.)

This patch allows C unions to have their structures copied or their instance property methods generated. Nested structures are also handled properly. After this PR, a compound structure like

typedef struct SDL_GameControllerButtonBind
{
    SDL_GameControllerBindType bindType;

    union
    {
        int button;
        int axis;
        struct {
            int hat;
            int hat_mask;
        } hat;
    } value;

} SDL_GameControllerButtonBind;

will generate

lib Binding
  struct SDLGameControllerButtonBind
    bind_type : SDLGameControllerBindType
    value : SDLGameControllerButtonBind_Unnamed0
  end
  union SDLGameControllerButtonBind_Unnamed0
    button : Int32
    axis : Int32
    hat : SDLGameControllerButtonBind_Unnamed0_Unnamed0
  end
  struct SDLGameControllerButtonBind_Unnamed0_Unnamed0
    hat : Int32
    hat_mask : Int32
  end
end

enum SDLGameControllerBindType
  # ...
end

This PR doesn't change how Crystal wrappers are generated for anonymous types, so if SDL_GameControllerButtonBind is wrapped, the only property methods available would be #bind_type and #bind_type=, as value will be ignored and its members cannot be inlined.

[HertzDevil]

Both done.

[Papierkorb]

LGTM, Thanks! :+1: