spirv: sim: adjust so Step{E,H}Context does not use ArrayHandle

the specific way to accomplish this is touchy.
see <https://github.com/EmbarkStudios/rust-gpu/issues/312#issuecomment-738824131>:

> So I'd say placing any of the spirv_std::storage_class types into an aggregate (including capturing it in a closure) is unsupported for now

in our specific case, we can't return a tuple where one element is a `&`
to a spirv Input, and another element is a `&mut` to a spirv Output.

when we have a struct, it can enclose either ONLY inputs,
or ONLY outputs -- not a mix.

i'm not 100% on how the Serialized stuff works, since it appears to
violate that. i guess that's exactly what this ArrayHandle stuff
achieves though.

crates/spirv_backend/src/support.rs

@@ -211,6 +211,13 @@ impl<'a, T> Array3<'a, T> {
             self.data.get_handle(idx)
         }
     }
+
+    pub fn into_ref(self, idx: Vec3u) -> &'a T {
+        let idx = checked_index(idx, self.dim).unwrap();
+        unsafe {
+            self.data.index_ref(idx)
+        }
+    }
 }
 
 impl<'a, T: Copy + Default> Array3<'a, T> {