| SubjectCopy | By default, Subject can only emit item which implemented Copy,
that because subject is a multicast stream, need pass item to multi
subscriber.
But Copy bound is not perfect to subject. Like &mut T should be emitted
by Subject, because Subject just immediately pass it to subscribers and
never alias it, but &mut T is not Copy.
So we introduced SubjectCopy, it's only use to support Item be passed
to many subscriber in Subject. The drawback is SubjectCopy is not auto
implement for a type when the type is implemented Copy. Just use macro
impl_subject_copy_for_copy! to impl SubjectCopy for your type after
implemented Copy.
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