Fix interface macro to handle qualified IUnknown parent

[rylev]

Fixes #1687

We have to differentiate between IUnknown and any other parent COM interface. This is because IUnknown::new only have two generic parameters (T: IUknownImpl and OFFSET) while all other interfaces have new function that takes three generic parameters (Identify: IUnknownImpl, Impl the interface impl, and OFFSET).

This change makes our check for IUnknown a bit more sophisticated. However, this is still a syntactic check and if a ident that is semantically equivalent to IUnknown (but syntactically equivalent) is provided (e.g., type MyIUnknown = windows::core::IUnknown or use core::windows::IUnknown as MyIUnknown), this check will not detect that it's IUnknown and the user will get the bad error message again which just says we're providing three generic arguments to new when two are expected.

The way to actually fix this is to not special case IUnknown and treat it like any other parent interface, but that's a bigger change that will require moving other things around.

[rylev]

FWIW: I looked into trying to make IUnknown behave just like any other parent interface and it's not quite possible, because we need to be able to prevent constraints on Impl traits from requiring that the implementor also implement IUnknown. Essentially we want COM classes to provide an implementation for all the COM interfaces except for IUnknown which the wrapper object _Impl struct implements for us.

Here's a branch where I attempt to make the change but run into the issue above.

[kennykerr]

Yeah, I'm finding generic programming in Rust to be a lot harder than C++. 😄 While somewhat lossy, this solution should suffice for the time being. Thanks!