Data races in multiqueue2

Affected versions of this crate unconditionally implemented Send for types used in queue implementations (InnerSend<RW, T>, InnerRecv<RW, T>, FutInnerSend<RW, T>, FutInnerRecv<RW, T>).

This allows users to send non-Send types to other threads, which can lead to data race bugs or other undefined behavior.

The flaw was corrected in v0.1.7 by adding T: Send bound to to the Send impl of four data types explained above.