Add AgileReference

[Nerixyz]

This is my attempt at adding an AgileReference to send references across threads, fixes #236.

AgileReference is generic over the type it's wrapping as this ensures the type wrapped remains the same.

In AgileReference::resolve I wasn't too sure how to return the correct type. I made the pointer, IUnknown is wrapping, public for the module, so i can construct a new instance (maybe transmuting would be fine as well, as long as ptr isn't null).
It returns the result of the cast. I can't directly transmute to the T since the size of T isn't known (even though it should always be the size of a pointer).

A better test and documentation is still missing.

[microsoft-cla]

All CLA requirements met.

[kennykerr]

Thanks @Nerixyz! I'll take a closer look soon when I have a moment.

[Nerixyz]

I'd like to add an example to the docs but I'm not sure how to do it. Specifically, I'd like to replicate the C#/WinRT example.

This is the example:

use windows::{core::AgileReference, UI::Popups::PopupMenu};

let non_agile = PopupMenu::new().unwrap();
let agile_reference = AgileReference::new(&non_agile).unwrap();

std::thread::spawn(move || {
    let popup = agile_reference.resolve().unwrap();
});

This doesn't compile, because the UI_Popups feature isn't enabled.

[kennykerr]

#1429 discusses the same problem. I'm not yet sure how to resolve that, but let's leave the example for now. I have to think a bit more about how to tackle documentation.

[kennykerr]

Looks great - thanks!