Improve enum type support

Currently only support C-like enums or consisting of unit types. SPIR-V (shader) doesn't support union types (afaik), so we probably will use a simple struct layout for emulating it:

enum Foo {
    A(u32),
    B(f32),
    C,
}

would be translated to

struct FooA(u32);
struct FooB(u32);
enum FooTy { A, B, C };

struct Foo {
   ty: FooTy,
   a: FooA,
   b: FooB,
}

This will need alot more memory than an corresponding enum, so complex enum types should be avoided.

Alternative

Keep the current state and disallow complex enum types, but add special handling for Option<T>.

msiglreith / inspirv-rust

Improve enum type support #4

Alternative