Refactor metadata and internal data structures

Replace Metadata.h with Containers.h, introducing a HashTable
implementation with SIMD acceleration and specialized track tracking.
Cleanup PlayerEngine logic by merging it into the playback flow and
refactoring several internal API signatures.

include/PlaybackEngine.h

@@ -28,7 +28,7 @@ class PlaybackEngine {
     void seek ( ma_uint64 frame );
     int isFinished () const;
 
-    ma_uint64 getPosition ();
+    uint64_t getPosition ();
 
   private:
     ma_device device_;