Draft support for VARIANT and PROPVARIANT

[kennykerr]

Hey folks, finally spending some time on VARIANT and PROPVARIANT. This is not quite done but do give me some feedback on the shape of things. Does this seem useful? Exhaustive coverage of the variant types may not be practical, but would this still be useful with partial support?

Fixes: #539

[mwcampbell]

To replace AccessKit's current code for constructing VARIANTs, this would also need to support VT_UNKNOWN. I suppose the next step is to make the generated bindings use the new VARIANT wrapper rather than the raw struct, making sure to get ownership transfer right, particularly when a method in a user-implemented interface returns a VARIANT, as with IRawElementProviderSimple::GetPropertyValue which AccessKit implements.

[mwcampbell]

I forgot to say, thanks for this and for all your work on windows-rs. I have no immediate need for this, as I already wrote the ugly code to create VARIANTs in AccessKit, but I'm happy to see safe and convenient support for more aspects of the Windows API.

[kennykerr]

Thanks Matt, yes VT_UNKNOWN support is on my list.

[kennykerr]

Final PR coming soon.

[kennykerr]

Final PR is here: #2786