Soundness issue: Input<R> can be misused to create data race to an object

Input<R> implements Send without requiring R: Send.

Affected versions of this crate allows users to send non-Send types to other threads,
which can lead to undefined behavior such as data race and memory corruption.

The flaw was corrected in version 0.5.1 by adding R: Send bound to the Send impl of Input<R>.