TryMutex<T> allows sending non-Send type across threads

Affected versions of this crate unconditionally implemented Sync trait for TryMutex<T> type.

This allows users to put non-Send T type in TryMutex and send it to another thread, which can cause a data race.

The flaw was corrected in the 0.3.0 release by adding T: Send bound for the Sync trait implementation.