os: implement waitable management.

This implements waitable management for Events (and
implements Events). It also refactors PM to use new
Event/Waitable semantics, and also adds STS_ASSERT
as a macro for asserting a boolean expression. The
rest of stratosphere has been refactored to use
STS_ASSERT whenever possible.

stratosphere/fatal/source/fatal_config.hpp

@@ -90,7 +90,8 @@ namespace sts::fatal::srv {
             }
     };
 
-    IEvent *GetFatalDirtyEvent();
+    os::WaitableHolder *GetFatalDirtyWaitableHolder();
+    void OnFatalDirtyEvent();
     const FatalConfig &GetFatalConfig();
 
 }