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/lib.rs

@@ -39,10 +39,9 @@ fn step_h<R: Real, M: Material<R>>(
 ) {
     if id.x() < meta.dim.x() && id.y() < meta.dim.y() && id.z() < meta.dim.z() {
         let sim_state = SerializedStepH::new(meta, stimulus_h, material, e, h, m);
-        let (update_state, mut out_h, mut out_m) = sim_state.index(id);
+        let update_state = sim_state.index(id);
         let (new_h, new_m) = update_state.step_h();
-        out_h.write(new_h);
-        out_m.write(new_m);
+        sim_state.write_output(id, new_h, new_m);
     }
 }
 
@@ -57,9 +56,9 @@ fn step_e<R: Real, M: Material<R>>(
     if id.x() < meta.dim.x() && id.y() < meta.dim.y() && id.z() < meta.dim.z() {
         let sim_state = SerializedStepE::new(meta, stimulus_e, material, e, h);
 
-        let (update_state, mut out_e) = sim_state.index(id);
+        let update_state = sim_state.index(id);
         let new_e = update_state.step_e();
-        out_e.write(new_e);
+        sim_state.write_output(id, new_e);
     }
 }