Future<T> lacks bounds on Send and Sync.

tiny_future contains a light-weight implementation of Futures. The Future
type it has lacked bound on its Send and Sync traits.

This allows for a bug where non-thread safe types such as Cell can be used in
Futures and cause data races in concurrent programs.

The flaw was corrected in commit c791919 by adding trait bounds to Future’s
Send and Sync.