[interpreter] Return instance wrapper from instance() (WebAssembly#126)

This doesn't change much currently, but the instance wrapper can be used in the future to send messages to worker threads, so they can remain in sync with the main thread.

interpreter/script/js.ml

@@ -30,8 +30,8 @@ let handler = {
 };
 let registry = new Proxy({spectest}, handler);
 
-function register(name, instance) {
-  registry[name] = instance.exports;
+function register(name, instanceObj) {
+  registry[name] = instanceObj.instance.exports;
 }
 
 function module(bytes, valid = true) {
@@ -53,20 +53,22 @@ function module(bytes, valid = true) {
 }
 
 function instance(bytes, imports = registry) {
-  return new WebAssembly.Instance(module(bytes), imports);
+  const mod = module(bytes);
+  const instance = new WebAssembly.Instance(mod, imports);
+  return {module: mod, instance};
 }
 
-function call(instance, name, args) {
-  return instance.exports[name](...args);
+function call(instanceObj, name, args) {
+  return instanceObj.instance.exports[name](...args);
 }
 
-function get(instance, name) {
-  let v = instance.exports[name];
+function get(instanceObj, name) {
+  let v = instanceObj.instance.exports[name];
   return (v instanceof WebAssembly.Global) ? v.value : v;
 }
 
-function exports(name, instance) {
-  return {[name]: instance.exports};
+function exports(name, instanceObj) {
+  return {[name]: instanceObj.instance.exports};
 }
 
 function run(action) {