Mutex and other resources are not destroyed

While looking at the Mutex implementation, I noticed that there is never a call to `pthread_mutex_destroy`, and the same applies also to other primitives. This "might" lead to leaks in the application. Might, because on some platforms that isn't necessary.

Now the current `Mutex::new` and `Mutex:: from_existing` results probably can't do that automatic freeing, since they are not sure whether something else still uses them. There might need to be either an `unsafe` destroy method for it.

Alternatively, there can be refcount stored in shared memory next to the mutex, and dropping any of those objects decreases it.