AsyncWorker: introduce Destroy() method

`AsyncWorker` contained the assumption that instances of its subclasses
were allocated using `new`, because it unconditionally destroyed
instances using `delete`.

This change replaces the call to `delete` with a call to a protected
instance method `Destroy()`, which can be overridden by subclasses.
This ensures that users can employ their own allocators when
creating `AsyncWorker` subclass instances because they can override
the `Destroy()` method to use their deallocator of choice.

Re: nodejs/node-addon-api#231 (comment)
PR-URL: nodejs/node-addon-api#488
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/async_worker.md

@@ -125,6 +125,19 @@ class was created, passing in the error as the first parameter.
 virtual void Napi::AsyncWorker::OnError(const Napi::Error& e);
 ```
 
+### Destroy
+
+This method is invoked when the instance must be deallocated. If
+`SuppressDestruct()` was not called then this method will be called after either
+`OnError()` or `OnOK()` complete. The default implementation of this method
+causes the instance to delete itself using the `delete` operator. The method is
+provided so as to ensure that instances allocated by means other than the `new`
+operator can be deallocated upon work completion.
+
+```cpp
+virtual void Napi::AsyncWorker::Destroy();
+```
+
 ### Constructor
 
 Creates a new `Napi::AsyncWorker`.

napi-inl.h

@@ -3529,6 +3529,10 @@ inline AsyncWorker::~AsyncWorker() {
   }
 }
 
+inline void AsyncWorker::Destroy() {
+  delete this;
+}
+
 inline AsyncWorker::AsyncWorker(AsyncWorker&& other) {
   _env = other._env;
   other._env = nullptr;
@@ -3623,7 +3627,7 @@ inline void AsyncWorker::OnWorkComplete(
     });
   }
   if (!self->_suppress_destruct) {
-    delete self;
+    self->Destroy();
   }
 }
 

napi.h

@@ -1803,6 +1803,7 @@ namespace Napi {
     virtual void Execute() = 0;
     virtual void OnOK();
     virtual void OnError(const Error& e);
+    virtual void Destroy();
 
     void SetError(const std::string& error);