On Windows there is no check if CreateMutex call failed for other than "already exists" reason but the code assumes we are running a single instance in this case which could be incorrect. A better solution is to return an error to the caller.
Added Send/Sync marker trait implementation so SingleInstance can be used in async contexts, issue #8.
This PR addresses the following issues:
CreateMutexcall failed for other than "already exists" reason but the code assumes we are running a single instance in this case which could be incorrect. A better solution is to return an error to the caller.