Make CoCreateInstance usable with Intercom

[Rantanen]

CoCreateInstance isn't that important for us, as the primary Windows interoperability API is winapi-rs. However there are some Windows COM APIs that we would like to use internally and pulling the whole winapi-rs as a dependency while we already implement most of the needed bits ourselves feels a bit silly.

The biggest issues for supporting external COM APIs are:

• CoCreateInstance support. This is rather trivial to implement for ComItf::new for example.
• Data types. For now we've mostly been going with the "We support only what we support" approach, which works fine for new APIs that have their primary definition in Intercom itself. However if we want to use existing APIs, we need to support the data types these APIs support. The two big issues on this front are SAFEARRAY (Support collection data structures #8) and various string types (Support other string types #27).

[Rantanen]

This was implemented in f790267, however the ComItf is not a good place for this. It should be implemented on ComRc instead - but unfortunately ComRc does not support these bits yet. \o/