Detect leaks in HandleAllocator object when ASSERTIONS are enabled.

Seems like it could be good to detect such leak so we could warn users if they, for example, forget to call emscripten_promise_destroy. Or do we want to just a silently ignore this?

I took a stab at it but ran into the issue that `HandleAllocator.allocated.length` doesn't change when things get deallocated.

```
diff --git a/src/library_promise.js b/src/library_promise.js
index 8de8ad9db..bf08f3ea0 100644
--- a/src/library_promise.js
+++ b/src/library_promise.js
@@ -15,6 +15,18 @@ mergeInto(LibraryManager.library, {
emscripten_promise_create__deps: ['$promiseMap'],
emscripten_promise_create__sig: 'p',
emscripten_promise_create: function() {
+#if ASSERTIONS
+ if (!promiseMap.atexit) {
+ promiseMap.atexit = () => {
+ if (promiseMap.allocated.length != 0) {
+ err('Leaked promises: ' + promiseMap.allocated.length);
+ err(promiseMap.allocated.keys().next().value);
+ abort('leaked promise handles');
+ }
+ };
+ __ATEXIT__.push(promiseMap.atexit);
+ }
+#endif
var promiseInfo = {};
promiseInfo.promise = new Promise((resolve, reject) => {
promiseInfo.reject = reject;
```