`Demuxer` can carry non-Send types across thread boundaries

In the affected versions of this crate, Demuxer<T> unconditionally implemented Send with no trait bounds on T.

This allows sending a non-Send type T across thread boundaries, which can cause undefined behavior like unlocking a mutex from a thread that didn’t lock the mutex, or memory corruption from data race.

The flaw was corrected in commit 0562cbf by adding a T: Send bound to the Send impl for Demuxer<T>.