Generate structs as Sendable

[tristanlabelle]

Structs are Sendable by definition:

Structs are always passed and returned by value. Struct fields may only be primitives, enums, structs, strings, and IReference (the latter two being the only two heap-allocated field types).
https://learn.microsoft.com/en-us/uwp/winrt-cref/winrt-type-system#structs

Not written on that doc page, but validated by midlrt is that IReference<T>'s in structs must be to value types, not to delegates, so the transitive closure of all types referenced by structs will always be sendable.

     delegate void SomeDelegate();
        struct OhNoNotInHere
        {
            Windows.Foundation.IReference<SomeDelegate> DelegateReference;
        };

test_component.idl(483) : error MIDL5105 : [msg]T in IReference<T> must be a value type when IReference is used as a struct field. [context]: test_component.SomeDelegate [ Field 'DelegateReference' of Struct 'test_component.OhNoNotInHere'  ]

Fixes #162

[jeffdav]

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