allow ZST entry params in input and output positions

[Firestar99]

With ZST entry params, you can do some cursed generic entry point stuff to make certain inputs and outputs optional:

rust-gpu/tests/compiletests/ui/spirv-attr/entry_generic.rs

Lines 19 to 28 in 5f1d97f

	#[spirv(vertex)]
	pub fn main<T: Copy>(in1: T, out1: &mut T) {
	*out1 = in1;
	}
	pub fn dummy() {
	main::<u32>(0, &mut 0);
	main::<f32>(0., &mut 0.);
	main::<()>((), &mut ());
	}

[Ralith]

As a prospective user, I think having entry points appear as a side-effect of monomorphization is potentially very difficult to reason about. I'd rather just write out the entry points I want.