add slice cast support

[39ali]

fixes casting array as a slice , or array to ptr :

*[T; N] -> *[T]

*[T; N] -> *T

fixes #465

[Firestar99]

Rebased it on main, squashed the fmt fixes and added disassembly and arrays from consts to the compiletest.

Overall, I'm super impressed how simple this has turned out to be. I haven't looked at our existing pointer emulation too deeply before, but this seems quite logical.

@LegNeato But this also shows that I've been wrong about RuntimeArray only being used via spirv_std::RuntimeArray, it is also used to represent slices. Just that those slices get removed in the post-link legalization, since they contain a pointer, so you don't actually see them in the disassembly of the compiletest.
See:

rust-gpu/crates/rustc_codegen_spirv/src/abi.rs

Lines 637 to 646 in bdf8bae

	FieldsShape::Array { stride, count } => {
	let element_type = ty.field(cx, 0).spirv_type(span, cx);
	if ty.is_unsized() {
	// There's a potential for this array to be sized, but the element to be unsized, e.g. `[[u8]; 5]`.
	// However, I think rust disallows all these cases, so assert this here.
	assert_eq!(count, 0);
	SpirvType::RuntimeArray {
	element: element_type,
	}
	.def(span, cx)

[39ali]

@Firestar99 we should probably use LogicalPtrCast in the future and resolve it in qptr, but the issue it'll get zombied since i think qptr is not on by default