Make RwLockReadGuard covariant

And refactor `RwLock` implementation into
non-generic core and generic interface

src/mutex.rs

@@ -163,6 +163,19 @@ impl<T: ?Sized> Mutex<T> {
     pub fn get_mut(&mut self) -> &mut T {
         unsafe { &mut *self.data.get() }
     }
+
+    /// Unlocks the mutex directly.
+    ///
+    /// # Safety
+    ///
+    /// This function is intended to be used only in the case where the mutex is locked,
+    /// and the guard is subsequently forgotten. Calling this while you don't hold a lock
+    /// on the mutex will likely lead to UB.
+    pub(crate) unsafe fn unlock_unchecked(&self) {
+        // Remove the last bit and notify a waiting lock operation.
+        self.state.fetch_sub(1, Ordering::Release);
+        self.lock_ops.notify(1);
+    }
 }
 
 impl<T: ?Sized> Mutex<T> {