Implement reference counting (#592)

lib/rtrace/README.md

@@ -0,0 +1,26 @@
+# ![AS](https://avatars1.githubusercontent.com/u/28916798?s=48) rtrace
+
+A tiny utility that records allocations, retains, releases and frees performed by the runtime and emits an error if something is off. Also checks for leaks.
+
+Instructions
+------------
+
+Compile your module that uses the full or half runtime with `-use ASC_RTRACE=1` and include an instance of this module as the import named `rtrace`.
+
+```js
+var rtr = rtrace(e => {
+  // handle error
+});
+
+WebAssembly.instantiate(..., { rtrace: rtr, ... });
+...
+
+if (rtr.active) {
+  let leakCount = rtr.check();
+  if (leakCount) {
+    // handle error
+  }
+}
+```
+
+Note that references retained in globals which are not cleared before execution concludes appear as leaks, including their inner members. A TypedArray would leak itself and its backing ArrayBuffer in this case for example. This is perfectly normal and clearing all globals avoids this.