Allow deriving `VectorOrScalar`

[nazar-pc]

The trait is unsafe and currently says:

Implementing this trait on non-scalar or non-vector types may break assumptions about other unsafe code, and should not be done.

Fair enough, but not particularly useful for someone who would like to implement it.

It'd be great if there was a way to derive its implementation with a derive macro. Very helpful for new types, structs with two numeric fields, etc. Without this capability things like subgroup_shuffle() currently require substantial boilerplate.

[Firestar99]

Luckily, #380 get us 90% there already. Just need to wrap that in a derive attribute macro that also implements spirv_std::vector::Vector and spirv_std::vector::VectorOrScalar.

See #411

[nazar-pc]

BTW, would it be difficult to implement this trait on things like [u32; 2] or more generally <const N: usize> [T; N] where T: Scalar?
It'd simplify some of the code I'm writing a lot.

[Firestar99]

[u32; 2] is an OpTypeArray %u32 2, not an OpTypeVector %u32 2, so it wouldn't work unless we change arrays to vectors for small array sizes.

I've been thinking about maybe a trait + derive macro specifically for subgroup intrinsics that would allow decomposing arbitrary types into the plain primitives and then apply the operation for each of those. That should be relatively easy to implement, next to what #411 offers. That would also solve the array case, since it's just a bunch of primitives.

[Firestar99]

@nazar-pc Made you #441 with a new trait ScalarOrVectorComposite:

rust-gpu/tests/compiletests/ui/arch/subgroup/subgroup_composite.rs

Lines 10 to 46 in ce822f1

	#[derive(Copy, Clone, ScalarOrVectorComposite)]
	pub struct MyStruct {
	a: f32,
	b: UVec3,
	c: Nested,
	d: Zst,
	}
	#[derive(Copy, Clone, ScalarOrVectorComposite)]
	pub struct Nested(i32);
	#[derive(Copy, Clone, ScalarOrVectorComposite)]
	pub struct Zst;
	#[spirv(compute(threads(32)))]
	pub fn main(
	#[spirv(local_invocation_index)] inv_id: UVec3,
	#[spirv(descriptor_set = 0, binding = 0, storage_buffer)] output: &mut UVec3,
	) {
	unsafe {
	let my_struct = MyStruct {
	a: 1.,
	b: inv_id,
	c: Nested(-42),
	d: Zst,
	};
	let mut out = UVec3::ZERO;
	out += subgroup_broadcast(my_struct, 19).b;
	out += subgroup_broadcast_first(my_struct).b;
	out += subgroup_shuffle(my_struct, 2).b;
	out += subgroup_shuffle_xor(my_struct, 4).b;
	out += subgroup_shuffle_up(my_struct, 5).b;
	out += subgroup_shuffle_down(my_struct, 7).b;
	*output = out;
	}
	}

Just be careful of the breaking changes in #380 and #440