Automatically dispose COM and WinRT objects with NativeFinalizer

[halildurmus]

Fixes #443

Attaches NativeFinalizer to IUnknown objects so that their pointers can be freed automatically by the NativeFinalizer. Note that freeing their pointers is not enough, we actually need to call release() on objects before freeing their pointers so that the resources on the native side can be freed as well. For example, here is a snippet from windows-rs that implements the Drop trait for IUnknown interface.

In order to do this automatically with the NativeFinalizer, we need to write a small function in C++ and use it as the NativeFinalizer's callback. The function is going to be something like this:

#include <windows.h>

#define EXPORT extern "C" __declspec(dllexport)

struct COMObject
{
	LPVOID lpVtbl;
};

EXPORT void DisposeComObject(COMObject *com_object)
{
	IUnknown *unknown = reinterpret_cast<IUnknown *>(com_object->lpVtbl);
	unknown->Release();
	CoTaskMemFree(com_object);
}

Bundling native code with Dart packages is a huge pain at the moment. Fortunately, there is an upcoming feature to make this process seamless. Once it is released, we should be able to easily use the above function as a callback for the NativeFinalizer and get rid of the calls to release(), and make disposing COM and WinRT objects completely automatic (yay!).

Also made some refactor/cleanup in various places.

[timsneath]

A few comments prior to merging; hopefully they're helpful!

[timsneath]

Great! Let's do this! :) Thank you.