add runtime pinner to pin unsafe pointer

[changchaishi]

Fixes #74

Hi @jchv,

I'm unsure if this is how you wanted it, but I tried to add PinHandler for the handlers. I did not use function wrapping to avoid handling the type assertion.

PS: I am new to go.

[jchv]

Thanks for taking a look at this. I am a bit too busy to properly review this, but I think this isn't quite ready to merge yet.

Firstly, the PinHanler function doesn't seem very useful. The calls to it may as well just call e.pinner.Pin directly instead.

Secondly, there's no call to e.pinner.Unpin. Leaking a Pinner will cause panics.

Finally, it might be better if there is some more general way to handle the runtime.Pinner for COM objects. I was thinking about the possibility of having each COM object have its own runtime.Pinner and call the Unpin when the refcount reaches zero. (Right now, I don't think we handle COM refcounting.) That said, there is a fair bit of nuance here so I haven't thought through all of the consequences. If you want to give it a go, though, feel free.

It isn't too easy to do automated testing with this library but we might need to find a way. Some of these behaviors (e.g. not panicking on GC finalization) could and probably should be tested, but there are no suitable tests to catch it yet.

[jchv]

(sidenote: the lint error indicates that I need to update CI.)

[changchaishi]

I am not ready for this kind of change. I close the PR first.