Unmarshaled COM 'interface' struct trimmability

[AArnott]

This change does two things:

1. Exposes the Vtbl struct itself. This may be useful for CCW generation, although that is outside the scope of CsWin32 (at present, at least.)
2. Makes the generated code more trimmable by not using the Vtbl struct at all. Each method directly indexes into the vtbl by pointers and then casts the desired pointer to the appropriate unmanaged delegate for invocation.

For example, here is the effective code change:

 internal uint AddRef()
 {
     fixed (ISpellChecker* pThis = &this)
-        return lpVtbl->AddRef_2(pThis);
+        return ((delegate *unmanaged [Stdcall]<IUnknown_unmanaged*,uint>)lpVtbl[1])(pThis);
 }

-private Vtbl* lpVtbl;
+private void** lpVtbl;

-private struct Vtbl
+internal struct Vtbl

This PR has no effect on generated code when allowMarshaling: true.

Closes #668