Implement :struct options for properties.

[marshall-lee]

Hello, @apotonick! I really appreciate stuff you build (Reform, Roar, etc), it's really game-changing!

I read a post about Reform 2.0 where you mentioned :struct option for properties which I found very useful in my tasks. However, I haven't found it implemented nor in Reform neither in Disposable. This patch fills this gap!

[marshall-lee]

The problem here is that Disposable::Twin::Struct adds :sync method assuming that class already got Sync feature. Maybe I should propose this change to Reform::Form that has Sync already?

[apotonick]

Thanks! :grin:

The Sync part is not a big problem, and this should go into Disposable as Reform is form-only.

We should also add a test that tests the struct: true. I still want to keep the old tests, though.