Improve unresolvable TypeRef handling (System.Guid)

[dagood]

Some types aren't in the current module, like System.Guid. Currently the tool logs them so the dev can implement them in another Go file in the package, but we could do better:

• Put the list of unresolvable types at the top of the generated file (for more visibility).
• Write code that explicitly checks that the type is implemented.

Resolving the types (across multiple winmd files?) is best, but I think some basic types will always need some manually defined types.